My friend you are the victim of ethanol! Ethanol wants to take on (absorb) water. You need to look at your check valve on your vent. They make a positive shut off check valve that goes on that vent line that will not allow any air back in. What happens is during the day when it's hot the gases expand and when it cools down they contract and it causes the tank to breathe. Thus it sucks in the humidity and knocks out into your tank because of ETHANOL!
You need to locate where your vent line is, for starters. It should not be located inside the hull. If that is the case you need to immediately move it to a place on the the rear transom but inside your boat. Buy a decent check valve, in line type and Viola. Your done.............. No more water in the tank. PS you must live where it's pretty humid, huh? Also every other tank I run a 16 oz bottle of SeaFoam (sp) through it to knock out the water. I guess it allows your filter to catch it. Remember, ethanol allows the water to thoroughly mix together with the gasloine sort of like a mixed drink the waay it does with alcohol. Capish?