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We own a 1983 GTX-18 I/O and I have done all the maintenance on the boat since the mid 90s. I can tell you that there is a channel from the gas tank compartment to the engine's bilge. Likely the boat would have to be at a pretty steep angle to get the last gallon out of the gas tank compartment. It was designed to contain the fuel in the event that you had a gas leak. My guess is there was some standing water in there to begin with.

I cannot say for sure how much fluid the gas tank compartment will hold as I have not taken the tank out of ours. I have removed the deck in order to replace the filler hose, the vent hose and the gas line to the engine. It seriously was not a big deal.

You don't want to mess with gas vapors and without inspecting the gas tank compartment, who knows how much fuel is there or what kind of shape the tank is in. I would at the least recommend removeing the deck and giving the tank and compartment a through inspection. You can only tell so much from the two access covers in the deck.

After removing the deck hopefully you and your brother will have the peace of mind to enjoy the rest of the summer rather than having that nagging feeling that something might be wrong. Be sure to get youself some sealer for when you put the deck back in place.

John