I might be in a similar sort of predicament, if I buy a boat in Canada
, then transit through the US or along the Eastern Seaboard to the Gulf to exit US waters.
The no visa 90 day arrangement (in my case) won't cover eventualities, so what I plan to do (so far, this may have to change, but it seems good at the moment), is take a Red Ensign with me, buy the boat, register immediately with the UK Small Ships Register (25 quid for 5 years), print out the paperwork (may have to wait for my kid sister to express deliver the originals once received), do the necessary SSR changes (carved woodwork and painting), then use an over 90 day visa to transit the US, which will allow for delays for any necessary servicing, repairs
, antifouling, etc., before jumping offshore
in the Gulf.
This should get you started: