In the "good ole days", sailboats and other non-powered watercraft under 16' were included under your Homeowner's liability (typically $300,000 limit). That's changed quite a bit in recent years, and I wouldn't rely on that if you clobber another boater/fisher-person.

Liability is the big unknown (you don't know who will sue you for how much), so at the minimum, I'd buy as much liability protection as I could afford (that goes for auto liability, too!).

You know how much it would cost to fix/replace the boat, so you could sketch out a "cost/benefit" analysis on whether to insure the boat damage or not.