I did my own insurance shopping except on speciality items (planes, boats) where I felt it was better and easier to let a broker who knew that specialized field do the legwork. Some good brokers have been mentioned here and I'd recommend using them.
MTOA has also negotiated what appears to be an interesting arrangement with Geico:
As MTOA members, we get the Premier Endorsement, Assumed Liability, Waiver of Depreciation (for boats over 20 years), and TowBoat US towing at substantial discounts. And Geico guarantees MTOA members that our renewal premium will not go up by more than 4%. (After the 2017 hurricane season, we are seeing their rates go up by more than 30% in certain zip codes and a minimum of 10% everywhere else.)
Might be worth joining MTOA for $65/year just for that.