I have read extensively on this issue:
1) Empty the tanks as much as possible before storage. The quantity of water produced from condensation will be nowhere near the quantity you get from E10 phase separation in a full gas tank and the subsequent hydration of the free ethanol, the water vapor coming in through your tank vents. Remember, ethanol mixes very well with water in mixed drinks. Everclear can only be refined to 190 proof (95%) due to the strong attraction of ethanol to water. Sealing of tank vents is not recommended due to temperature fluctuations should pressure build up and force fuel into the engine or stress seals or joints.
2) Put in additive to the remaining fuel (Star-tron, Sta-Bil) or equivalent.
3) Change your water separating fuel filters and fill up with fresh gas before the engines are started after any extended storage period. You must remove the water from the bottom of the tank to avoid drawing liquid water into the engine. Severe engine damage will occur if you try to compress water. Disconnect fuel line and turn over engine to check for water if there are any doubts.
4) If the boat is trailerable consider filling the fuel tanks from a source that does not use ethanol before the storage period. You may still need to use a fuel stabilizer even with 100% gasoline.