I have seen places that say to take the tanks out, but this is likely not always possible. Others suggest only doing while boat is docked, etc. But, I did see the following:

Boat Owners Association of The United States
Know fuel tank capacity: To prevent overfilling
Never use your boat's gas gauge while filling up: If you are not sure of your tank's capacity, never turn on the vessel's ignition switch to view your boat's gas gauge. Sparking can sometimes occur.
Don't walk away
After half-full, slow down, reduce the chances of blowback.
If your boat is not level on its trailer you may find fuel inadvertently exiting the vent, or unable to fill tanks completely.
If you have two or more tanks, distribute fuel equally. Loading up one tank more than another could put your boat-and-trailer-combo dangerously out of balance.
Never top off: Fuel that is pumped from cool, below ground holding tanks into your boat's tank - warmed by hot asphalt below and bright sun above - will expand.

Mike Carlson, CFI, CFEI, CVFI, CATI, CPP,
Chicago, Il