For a boat in the 26-30' range, your car/home insurer may be the most competitive. Most won't cover them larger than that, and then your rateing will depend on things like the area you are in, the value of the boat, the age of the boat, the coverage you want (of course), and whether you are a first-time boat owner or can document prior boating experience and training.
For a bigger boat and solid coverage, yes, IMIS gets consistently high praise. Not cheap--but pros who deal straight.
Policies that look alike at first glance can also be very different, if you get two very different quotes don't be afraid to ask each one to compare line by line with the other, and what other coverage they recommend. (Like fuel spill and salvage liability.)