It's probably the type of boat you own rather than the length. Mainstream ins. co. like Allstate, State Farm, etc. usually won't insure performance boats.
Although, State Farm insured all the boats I've owned. After I purchased my latest boat SF told me that they would not insure it. They later changed their mind.
I'm thinking of switching to either Wakezone or Total Dollar. I've gotten pretty competitive quotes from both. I also got a decent quote from Boat US.
Check out the "Insurance and Finance" section, lots of info in there.
BTW, age and driving record can effect your rates too.