OUrs, the LPG model, was on a 28' Cape Dory. We used it for years in Vermont (Lake Champlain), then on the coast of Maine. After an hour or so, it would radiate some heat, but you had to be right on top of it! I remember some really cold-wet Maine fall days. I'd sit next to it rolled up in blankets. Eventually, my hands got warm again.
That heater was mounted on a bulkhead above a settee. Not ideal. And the boat had no insulation. Don't get me wrong though, you love any heater on a boat when you're cold; we used it a lot.