The purpose of surface treatment of a wooden boat is, at least, to:
Why should water not get into wood?
First of all, as explained earlier, wood swells and shrinks along with its moisture content. A wooden boat is full of structural details, where pieces of wood have been joined with crossing grain directions (as an example, every plank is fastened to every frame at both edges). A furniture carpenter would never build anything like that. Wooden furniture structures are always floating, allowing wood to swell and shrink freely as its moisture content changes. Floating structures have not been used in wooden boats for centuries, so a modern wooden boat breaks itself, if wood moisture content is allowed to fluctuate.
Secondly, wood rotting fungi can only act on wood, when wood moisture content is 25-30 % or more. In addition to that, fungi need warmth and "edible" wood.
Thirdly, wet wood is weaker than dry wood.
There is no way to prevent movement of moisture to and from wood totally. One can only slow the movement down. It would be ideal if the annual wetting-drying cycle (and shorter cycles) would be slow, small and would not drift either way (that is, the wood would not little by little get drier nor wetter than it was when the boat was built).
Water comes in two brands. As liquid and as vapor.
Water vapor gets absorbed in wood cell pipe walls, but does not under any circumstances cause the cell hollow to fill.
Liquid water gets absorbed in wood cell pipe walls, and can also fill the cell hollow.
Cell walls can absorb water up to 25-30 % of wood dry weight. The wood moisture content can thus rise to 25-30 % without any water in the cell hollow.
This can only happen, if the air relative humidity stays at 100 % for a long period of time. Air humidity does not stay this high for long periods in most natural environments. Water vapor will not make wood moist enough to maintain growth of rot, in most practical cases. Liquid water is needed to enable rot.
Liquid water can raise the moisture content of wood above 100 %, because water not only saturates cell walls, but also fills all empty space within the cell structure. Wet wood can thus contain more water than wood, by weight.
Water vapor penetrates all wood surfaces rather equally. Liquid water, again, penetrates wood especially well parallel to grain, at wood ends. Less so perpendicular to grain. This is clear from the "cell pipe model". Water enters a pipe from the ends, not through the wall.
Wood dries by emitting water vapor. Water in the wood vaporizes on the surface. If a surface treatment slows down the movement of water vapor, it slows down wood drying.
A given surface treatment does not necessarily prevent liquid and vapor water movement in a similar manner.
The ordinary surface treatments slow the movement of liquid water in the order epoxy-polyurethane-alkyd-oil. But the differences are not huge.
With vapor the order of materials is the same, but there is a step between alkyd paints and oli paints. Oil paints and linseed oil don't slow down the movements of water vapor much. Some say oil treatment "breathes".
Wood gets wet, when it absorbs liquid water or water vapor.
Wood gets dry, when it emits water vapor.
A good boat paint should prevent both liquid and vapor water from getting
into wood, but should allow water vapor to exit freely.
A paint like that does not exist.
Epoxy and plyurethane paints do prevent water (both liquid and vapor) from getting into wood. But when (and I don't write "if") wood finally gets wet, it stays wet, since the paints also prevent vapor from getting out of wood.
Oil paints or treatment with linseed oil do allow wood to dry, but also do allow water vapor to moist wood.
Things being this way, the question "what kind of paint should a wooden boat be painted with?" cannot be answered unambiquously. Arguments both for and against can be given to all paint types.
Since wood and surface treatments behave as described, we can conclude:
The ends of every single piece of wood should be treated with a coating that prevents entry of liquid water. With epoxy during the boat construction, or with linseed oil once the boat is finished. Only linseed oil is running enough to penetrate the seams, that were closed during boat construction. At least in principle. Water will penetrate in practice. It has time on its side.
The worst possible alternative is to leave wood ends untreated, but cover the sides with something preventing water vapor flow. Wood will suck liquid water through the ends, and never dry. An example of this would be painting a ready made boat inside and out with epoxy.
"Varnish or paint on the outside, linseed oil on the inside." The old traditional approach is good. Varnish or paint keep the wood from absorbing water from the sea or lake, linseed oil allows the boat to dry from the inside.
Assuming, of course, that liquid water is kept out of the boat, and the boat interior is well ventilated.
This old "recipe" has another benefit. Cleaning, sanding and repainting boat interiors is typically horrible, if not worse. There are lots of holes, slits and corners. Re-oiling is a lot easier. There is no need to remove nor sand old oil, and oil is easily running and sperading.
The cell hollows in wood can be filled with hardening oils (called oil saturation) to leave water less space to dwell. Wood species that have closed cells may not saturate well. These species have, just because of the cells being losed, some natural tolerance against water, so the balance may be even.
Oil saturation underneath a paint or varnish is also a good combination, as oil prevents water from geting into wood when the paint surface gets scratched.
But it may also be harmful, if oil prevents paint from sticking to the wood.
A typical oil saturation recipe consists of 1/3 raw linseed oil, 1/3 thinner (pine turpentin is often recommended) and 1/3 liquid anti-rot compound. The anti-rot is mostly thinner. When wood cells have been saturated with a slution like this, the cell hollows are full of liquid. To start with. But as the thinner evaporates, only the 1/3 of linseed oil remains to harden
It would seem, that to reach a very good saturation result the saturation should be repeated several times, with enough time (days...) in between saturations. This would ensure cells filled with oil, not just evaportaing thinner. This procedure would not be very economical, of course.
Or to use a preparation with a higher oil content. What would be best? Remains to be investigated.
In case the surface treatment cannot keep the wood dry enough for rot not to grow, wood can be saturated with fungisides.
In a typical linseed oil mixture the anti-rot potion is a mixture of thinner (some 99 %), tolyle fluanide (typically < 1 %) and zink naphtenate (typically < 1 %). Tolyle fluanide is a fungiside, zink naphtenate an anti mold compound. The purpose of these is to poison the wood, but also poison the linseed oil, which also is food for fungi.
If You want to poison the wood separately, a sensible working method would be to apply ani-rot agent first, as such. Than let it dry properly before following treatments. Since anti-rot is mainly thinner, no furher poison nor oil can be absorbed by wood cells before the thinner has evaporated.
It is also possible to poison wood by ethylene glycol (anti freeze) or boron, which are water soluble. Unlike the thinner soluble anti-rot substances they go to where wood is wet. That is, to where fungi live. It is possible to kill existing fungi in wet wood with these water soluble poisons. Impossible with a thinner based poison.
The water solubility is also a problem, of course :-( These poisons get quickly washed away by water.
UV-radiation breaks down wood lignin. Remeber, the glue between wood fibers. When lignin breaks down, cellulose fibers turn loose on the wood surface. Wood is shattered little by little.
Opaque paints are best protection against UV-radiation. Most varnishes have some added UV-protection. UV-protection in varnish is finely ground pigment. Little enough to let wood show through the varnish. But enough to prevent at least some UV-radiation from reaching the wood surface.
UV-radiation also breaks down the surface treatment. Epoxy as such would be enough to protect wood, but epoxy itself must be protected by paint or varnish.
Paint gives a much better protection than varnish. A good paint may last for 15 years. A good varnish may last three. Varnish "bubbles off" the wood, because wood underneath it breaks loose.
This is, of course, largely a matter of taste. But You can't go very much wrong if You leave the boat interior and decks wood colored, outside of the hull wood colored, white, dark blue, dark green or black.
Although my father taught me, that a black boat brings bad luck.
When the car ferry Estonia sank, rescue helicopters had difficulty finding the capsized life rafts, because bottoms of the rafts were black.
A white, yellow or orange boat color may save Your life one day.
The bad luck brought by a black boat may have an explanation here. A black boat in trouble on a stormy sea may be hard to find. This, if anything, is bad for the crew, although luck has nothing to do with it.
A drunk power boat driver may see a light colored boat better than a dark colored. But can he steer clear?
Temperature changes expand and contract wood. Partly by thermal expansion, partly by changes in moisture content. Heating speeds up moisture evaporation. Wood expansion and contraction break the boat.
A dark surface may get hot enough to soften epoxy and elastic sealants. A maximum allowable temperature is typically in the range of 50-60 degrees centigrade / 120-140 Fahrenheit. Again, use light colors.