Paint below the waterline

Any bottom coat paint that you'd put on a sail or powerboat. I think that the anti fouling properties are the issue. I've never left boats in that long, so its speculation...
Nearly all enamels will be peeling within a week or two, so they're not a good choice. After trying a bunch of different brands and types of bottom paints on my sailboats over the years, I finally settled on an antifouling paint called "Hydrocoat". It's an ablative, meaning that it slowly dissolves away as it works. This means that you never need to go back in and sand off old, dead bottom paint (which is a thoroughly unpleasant and somewhat toxic job). You just keep an eye on it and if it needs a new coat or some touch-up in the spring, you slap some more on (not to be confused with slapping a moron :D). It's water-based, doesn't stink and goes on about like painting latex on a wall. Not cheap, but all good anti-fouling paints are expensive.

I do wonder though, about the durability of the canvas filler when left in the water for that long. Will it slowly soak up water (no paint will completely stop water absorption at the surface) and then peel off or dissolve? I have no clue, but I think it's well within the range of possible scenarios no matter how many morons you slap.