Just an update: I LOVE this Webasto system! It worked great all summer. I did swap the sea water strainer's basket, which was plastic, for a stainless one. It was almost totally blocked toward the end of the summer (it's tidal, salt water here on the lower James River and full of marine life). Other than that, and adding a third vent that I can direct toward the head or the main cabin, I've had to do very little. I've got it set to automatic operation now, so it heats at night and switches to dehumidify and then AC as the cabin warms during the day. The blower runs all the time, and is certainly not quiet, but I don't find it at all annoying. I may take some DB readings at some point, if I think of it. I did put some reflective foam insulation in the forward hatch, which comes out easily if I want to open things up. And having some shade over the companionway area is a big help during the hottest months. Generally speaking, though, the 9000 BTU capacity seems just about right. I have no trouble running it, the hot water heater, and the charger keeping three banks topped off, on 30A service.
I'm continually amazed at what a pleasure it is to get to the boat and have it always be dry and comfortable. Given how much time I spend working below decks, it is a real asset.