One of the issues with the Force 10/Sigma Cozy Cabin heater is that the combustion chamber is not sealed. This allows moisture (by-product of propane combustion) to escape into the cabin -- much like using your cooking stove/oven. For limited heating it's probably okay, but if you plan to operate the unit for long periods, you will begin to notice the dampness in the cabin (and condensate dripping from your cold, bronze portlight frames).
The Dickinson Newport heaters are more expensive than the Cozy Cabin units (by a factor of 2-3), but they have a sealed combustion chamber that exhausts all the moisture out of the cabin via a double-walled chimney (the interior flew exhausts the by-products of combustion, while the exterior flew brings air for combustion from outside the cabin and simultaneously cools/insulates the hot interior flew). This results in a nice drying heat.
Here is our Dickinson Newport propane cabin heater (not the greatest photo). It's the smaller of the two sizes offered (P9000). I would think on your boat the larger 12000 btu unit would be preferable, supplemented by a few electric heaters when dockside:
P.S. You may also want to consider some way of insulating your portlights if you are going to stay put in one place for the winter. I have seen various methods but one I liked was clear bubble wrap affixed to the exterior -- which still allowed light penteration.