Big question is how the air/raft is sabotaged. Quite a few things could still be usable, given the nature of the "damage." If you really want to give th e players a clever out, you could go Flight of the Phoenix and let them construct some kludgey grav belt so that one of them can go get help. Or something ...

Oh, and that was a fantastic description of an air/raft. You can also check out the T20 supplement Against Gravity.