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.