If the underwritter is overseas, your rate will be higher. But an overseas underwritter will insure a boat that an american one wont.
I've been told this is in part that most American underwriters are really setup for land vehicles and their marine part is so small they stay away from any risk, whereas the overseas ones are mainly or only marine centered.
Hey, the entire rest of the world uses what we would consider an "overseas" carrier, you may be surprised. But i really do not know either way.