This subject has been discussed several times in recent weeks. It boils down to not being able to register a boat with the CG if you are not a citizen. How you proceed depends upon what you ultimately wish to do with the boat. What flag do you wish to register the boat with? You can purchase
a boat, re-register it and then apply for the necessary cruising paperwork. Remember too, that you will need a tourist visa in order to sail into and out of the USA on a private vessel (the normal visa waivers do not apply!).