But TBF you're not supposed to sit on them, so that's a mistake of the user not the tank. And I'd doubt a small puff of fumes would cause any issue at all.
Same with the pressure venting. Ive only seen a plastic tank swell when left in direct sunlight, or in extra warm places. The bilge of a boat is not a particularly warm place. But it's like everything on a boat, check and maintain and you should be ok! Plastic fuel tanks are fitted to boats all the time, ok so they vent outside, but I can't see a 20ltr portable giving any issues if well cared for.