Also, you need to make sure your boat is really wired for a more-or-less continuous load and that your wiring is really okay.

Wires in a salt environment can corrode over time, so you have diminished capacity. The actual rating for continuous loads becomes more critical. If you're heaters are really on most of the time to keep you warm...

Most believe that it is better if your wiring is a quality marine grade, tinned wire -- but, even then, that's not forever. (For example: Taking 10% outside diameter of the wire size from corrosion would remove about 20% of the cross sectional area capacity; and, that's neglecting the added resistance from heating-up the wire if your overload it. I wouldn't count on the wire being a bit colder to start with than during the Summer ;^))))

If you're away from the boat, I'd consider running at 60% or so of rated capacity for the heaters that are on (including a hot water heater). If that breaker pops, it will take a while to heat the boat back-up. If you potable water systems aren't winterized, you can have issues. In building fit-outs, the breaker is designed to be a % of the wire capacity -- but on many boats, it's not treated that way.

Of course, the heaters in the boat will do little for some lines (like cockpit shower hoses or washdown pumps) -- I've spoken with some people that failed to take care of those issues and had to face the consequences.