There are many threads on this issue. Your bill of sale
will need an apostile or notarization.
The apostille is particular to ex-us purchases.
Search the forum for that word and you will find the threads.
There is also a lot of speculation and noise
in the threads. Pay attention to the posters who have actually USCG documented a boat
that was in foreign registry.
The NVDC is helpful.
I used a documentation service
to do this for me. It cost 600USD a few years back. Worth it.