Definitely a Racor-type water separator, check the gasket on the fill cap, and make sure your vent is not susceptible to runoff from surrounding areas. Of course, you also have to check the bowl on the filter to make sure you're not filling the filter with water (had a near-incident in Hawaii diving off an acquaintances Whaler; the filter canister was several years old, and filled with water, rust, and sludge. Enough to starve the motors of fuel. Removing the bowl and emptying/wiping out the foeign material got it running enough to get back to the harbor.)
Fuel stations use a flourescent dye to stain the water at the bottom of the tank, which then shows up on a dipstick, but that may not be an option depending on your fill tube configuration.
Perhaps regular use of a "Water-Zorb" product?