It sounds like a deathtrap and the best thing would be to let me take
the thing off your hands and dispose of it properly.
But if you insist on risking your life, I would put it in the water and
see what happens, you can always pull it out to remedy problems. It
would seem once the boat gets remoisterized those cracks will
disappear. In a crack that size, you would have trouble getting any
caulking in anyway. Maybe you could wrap the boat with plastic and keep
it wet for a day or two and see what happens. Put some humidity on the
inside also so it's more like sitting at anchor as opposed to sitting
in a shed.