Sean, I'd also only suggest a filled epoxy
. Filled with pretty much anything stable, clean sand, microspheres (hey, you might want to race<G>), anything bulky cheap
and impermeable. Throw in some glitter if you REALLY want to confuse the next owner.<VBG>
The cement-based fillers will always hold moisture (cement has a high water
content) and never bond really well, so there's another "crevice" under them. Polyester resin *might* bond well enough since this is not structural--I'd chance it only if a supplier guaranteed it would.
There are also some floor filling/sealing compounds that are hard to find, something like an acrylic
casting resin used to seal over fancy floors like the resin you see on some bars and restaurant tables? I don't know if that's suitable for damp places, or if that's any better/worse than just buying
polyester resin.<G> It won't be any easier to find.
If you can trim the boat stern-down before you start (really HEAVILY down) then when you are finished, the "flat" surfaces will have a slant slant to them--and drain forward. Or trim bow-down if you need the spaces to trim to the stern, whichever.