If you're going to be tied up at a dock where electricity is available, why not buy a decent quartz heater and just plug it in when you need it. It's a clean source of heat, requires no ventilation, and is inexpensive to buy. This will also allow you cut way down on your propane use.
If you're away from the dock, your propane heater should work quite well for you. Your propane stove is little different than the stove I'll be cooking on tonight and is almost maintenance-free. So, why change it out?
One way a diesel is supposed to be different from a propane stove is that the diesel can double as a cabin heater.
I should have mentioned I'm trying to be as off-grid as possible. I'm looking into solar panels and wind generators too. I'm not sure how much electric I'd use with a heater in a Canadian winter, but my impression is that electric heaters are the most wasteful or energy inefficient of all.
Of course, living on diesel is not exactly being "off-grid", is it...but at least it means I can go for long passages without needing to plug into shore power, which is what I'm aiming for.