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The worst thing you can do with a fuel tank is to foam it it! Carl Moesly knew better than to foam in a tank, but Potter evidently started doing it by the mid-70's. Second worst thing is not to coat it with a flexible coating like coal tar epoxy! As the tank expands and contracts, it'll create a small gap between foam and tank which will hold water against the aluminum, causing crevice corrosion in the bare aluminum! While you have the hatch off, I'd break out the pressure washer and blast all that foam away, then pull the tank and inspect it for leaks, or maybe have it pressure tested. Pulling a tank is relatively easy in the Seafari compared to a CC model! Then reinstall the tank per Dave Pasco's recommendations.
As for recoring the hatch, the first challenge will be to get the bow out of it as others have mentioned. One challenge in using foam core material is that resin doesn't penetrate it like it does balsa core, so it's harder to get a good bond all over the bottom side that you can't see, unless you use the vacuum bagging technique. Before you get too deep into the repair, check out Dave Pascoe's article on core materials. Balsa core is actually stronger than foam and bonds much better, so it's a great material provided you don't go drilling holes in it! A 1/2" core will be plenty stiff if you put a layer of 1708 over it, but I'd inspect the top of the hatch closely to see if you can determine how water got into the plywood, which deteriorates quickly if it gets wet because water follows the grain in all directions.
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Thanks for that Bushwacker. I'll look into all those details. Coating the tank is a great idea; I will take heed of your suggestion. My father replaced the tank in 2010. He had it fabricated locally from the original, which leaked out a full tank of gas before realizing the issue. I'm sure he replaced it in the same manor he found it, as you said originally, with the foam. I wish my fuel gauge worked currently to tell me how much fuel I'd need to remove; it works intermittently! Can you guide me on what/how I can check on the tank-end to rule out any possibilities there? The gauge pegs left past E when I turn on AUX. This is another thing I really need to correct before the boating season, in addition to the tach (again works intermittently). Like you say, while the hatch is open give everything there a clean bill of health. The wire connections are badly corroded.
With regards to the source of water, I'm thinking it is the screw holes; although parts of the edges were compromised also. The bottom fiberglass came off in one piece with just a little prying. I got my grinding wheel out, respirator, and ear/eye protection and didn't use any of them... yet. You can see that the original plywood goes right up to the edges, and is not beveled, like is recommended today.
Last night's progress:
Fiberglass bottom removed, along with some plywood...
1/2" plywood originally used:
I still have some scraping/grinding/sanding to do. Doesn't the plywood remnants look like bacon!? Some of the wood scrapes off and looks like pot roast and/or pulled pork. Can't you tell I worked straight past dinner time? LOL
The bow in the fiberglass is one thing. There are also "divots" in it, like heal marks or low spots from use. My wheels are spinning about how I can apply pressure and do so on a FLAT surface. I have a few thousand pounds of weights at home, and although "overboard", granite surface plates at work... I may get quotes from a pro with a vacuum table for the hell of it too.