I sure would like to know what the name of the insurance company is that sells homeowners policies that sell policies that will cover a boat, truck,atv, etc. that gets damaged on said property.
Your neighbor has a liability policy that pays for things they are liable for. The tree falling was not their fault (unless it was rotten). Therefore their insurance won't pay.
Your homeowners insurance has a limit for the size boat it will cover spelled out in the policy. Its often 25hp or under. If your agent knew you had a boat he shoudl have encouraged you to insure it. He could probably tell you on what date he offered it to you if he keeps good records. I'm sure you've taken the time to review all of your insurance with your agent. If you didn't then you're passing on an important feature of having an agent.
None of that makes it a scam.