To replace a boat fuel tank:
Shut off the fuel lines and hoses with vise grips. Drain the fuel from the old tank and dispose of it. Reconnect the hoses and turn on the engine to burn off any excess fuel. Let the tank cool, and then disconnect the hoses and fuel gauge sending unit.
Take out the safety brackets that connect the current tank to its mount. Lift the tank out and dispose of it.
Prepare the mounting point for the new tank. This may involve updating the deck with a new plywood base. Once the surface is prepared, position the new tank in place and fasten the brackets.
Reinstall the fill, vent and fuel hoses that are in good condition. If the old hoses are in poor condition, purchase new ones. Check that the hoses are secured at the fuel tank and the engine. Reattach the fuel gauge sending unit.
Pour fresh fuel into the tank until it reaches the recommended level. Turn on the engine and inspect the connections to ensure there are no leaks.