Hi Don,
I have two sail boats that I insure through State Farm. The coverage covers the replacement/repair cost of the trailer, boat, and motor along with sails and other miscellaneous things on each boat. Ones a 22 ft. Catalina (on a trailer), and the other is an O'Day 27 (a bit bigger). I think the per month cost per boat is in the $15 to $25 range, and the small boat with partners is the one that costs the most (even though the vaule of the boat is a 1/3 of the larger).
Our dive gear is covered by our renters insurance, and I'm not sure if boat insurance would cover the cost of dive gear.
I would suggest talking to a few insurance companies, get some quotes, and see what they cover. State Farm gave us a good deal on our coverage since we have cars, motorcycles, and renters insurance through them. There are other boat only services (U.S. Boats or West Marine) that also provide coverage, but for us, State Farm seemed like a better deal. The only thing that we don't have is towing insurance, but on power boats, it can come in handy.
Hope this helps.
~ Jason