Ethanol Fuels?

[FNQCairns]

Yeah they are not going to put a high pressure 60psi fuel pump very far from the engine on a boat (at least I hope not!!).
Pinhead you have a breather in some way, some how, just like outboard motors, more than most probably it is a thru hull that if breathed thru with the fuel cap off would spill fumes out.

Ifi t were a closed system every L your engine used the atmosphere would crush your tank by a L till it was very wrinkly indeed!

cheers fnq

PS that last statement is a bit of overkill but relative.