If you get a Racor water separating fuel filter canister with a see through plastic drainable sump, you can drain the water out of the filter periodically. They'll screw onto the typical fuel filter mount. Because the sump is plastic and can burn, they are not supposed to be installed in an enlosed bilge or unvented space. You can also get a metal sump to screw to the bottom of the cannister, but you can't see the water inside those unless you drain gas out. If your tank is old, it may be corroded and water is getting in that way. If that is the case, your bilge will stink of gas fumes and the boat is a bomb waiting to go off. Also check the gasket around the fuel filler. They can leak and take on water from rain or seas. Before you go through the trouble of pulling it out, pressure test if for bleed down with about 5-8 psi of air pressure. It's pretty easy to plug the vent hose and fuel lines while you do this. Napa sells a leakdown kit that will work with any compressor.