Thanks for the reply. I appreciate it. Do you know anything about this hull? It's stamped with "F20" but there's not much information that I have been able to find on it.
As for the water, it's definitely possible that I didn't get all the water the first time. I really hope that's what it is. Where the sending unit is, there was no gasket, either when I removed it or when I replaced it. When I removed it, there were bits of caulk and stuff on the bottom of the face of the sending unit that I figured was no good. I cleaned it up really well, making for a nice smooth contact surface. I screwed it back on, and sealed it up with a good amount of silicon. Hopefully the lack of a gasket will be overcome with the silicon. If not, I can remove it and add some sort of gasket.
I can see what you mean about the vent being placed in such a way that water could splash up and get in there. I can also see how it could be possible that someone put a screw into the floor that punctured / pierced the tank. Particularly where the leaning post is attached, that is something I'll need to check for sure.
Thanks for the ideas... I'll keep you posted.
Chris
P.S.
Here is a link to a (large) picture of the hullID