If it is in the high use season, you can get away without it, but if you leave your boat sitting on the high humidity waterfront and your fuel tank vent is open to the atmosphere, the daily temperature changes expand then contract the volume of fuel and outside air is induced into the tank, water will be accumalated in the tank due to the E10 alcohol and its affinity to suck up the water and then separate in the tank. The water is on the bottom and when you start up, you get a slug of water and you are stuck at the dock for your day off.