I am in the middle of rebuilding one of these boats that used to be my father's because the foam was waterlogged. I've replaced everything and went ahead and redid the wiring since it was already split in half but I'm having trouble finding out what the Grey sealer is they used to seal the top to the bottom. It's probably 40 years old, still malleable and tacky to the touch. If left in the sun it spreads out. Anyone know what this stuff is?

You might look at 3M 5200 Marine Sealant or one of their less permanent options. I know for a fact that 5200 works great at holding things together 

Sounds like Butyl tape, commonly used on rv's. It's a grey or black sticky putty like product that comes in a roll, it stays malleable. I'd not want a permanent bond as these boats tend to do like your dad's did. I've used butyl tape replacing windows and such on my old campers, I'd think it would work well for your project.

 

There's a couple Facebook groups for these, might join and ask the question there.