OK, I figured this out, so I thought I'd share it here in case anyone else needs it.
For documented
boats, the coast guard has a standard bill of
sale. For multiple owners in different locations, just print fill out and print two bills of sale, one for each owner (in this case, each owner owns 50% of boat). Have the two bills of sale notarized individually in each owner's location, and present both to the buyer, adding up to the 100%.
The new owner will send both forms in to the documentation center.
Easy peasy.