I'm in your area and my insurance went up a little bit this year too. It didn't increase at the rate that you have mentioned but it did go up. It does pay to shop around for boat insurance. You can do a search on the topic and get some names of people to try. I am through Allstate and have had a very positive relationship with them.
Where do you do most of your boating? Ever make it up to Madison, WI? I'm guessing you are on the Rock River?