It's very hard to compare insurance UNLESS (A) you're basing it solely on price, or (B) you've been through several claims with different companies.
Price isn't always the best comparison and few of us have been through multiple claims with different companies.
That said, I use BoatUS. I have been through two claims... one after a lower unit went up against a rock, and lost. The second when I was rear-ended on the highway and my trailer was totaled (miraculously, the boat was unharmed).
In both cases I was very satisfied with BoatUS and how quickly and well they responded.