how could this possibly happen

[WildBill]

Just another fun fact found out the hard way. My brother used to fish offshore in a smaller boat, a 19' Wahoo in rough sea's (an honest 5'-6') and his vent would get a little to close to the sea's if you catch my drift. He would ultimately suck in sea water through his fuel vent all day long. He would make it in and not notice anything was wrong but after a couple of trips and between sitting and refueling the problems would arise. We figured it out after a few years of fuel/water issue's. I'm assuming your talking about your 23' Sea Craft and not a smaller boat. But if you fish or travel through heavy sea's this could explain some of the water in your tank. You should know how close your vent opening is getting to the sea's once you think about it; and check which way its pointing (should be pointing aft and downward) the water tends to roll up the sides while running through the slop. It may or may not be whats causing your issue but just something I've experienced and thought I should share.
Good Luck
Wild Bill