I started with a quote from BoatUS and some of the other big names after researching a bit on sites like DiscoverBoating.com
that offer overviews of boat insurance. We ended up with Progressive because we have a really good rate with the multiple policy discount (auto too). We've been with them for years and have never had a boat claim, so I can't really rate them.
However, we'll probably shop around when we upgrade to the next boat since our cruising plans include long distances and much more variety in cruising grounds. Shopping based on rate probably isn't the best bet at that point. As you've found, smaller (and often more expensive) companies seem to offer more flexibility.