rsn
Your issue is the maximum of 15amps of hydro power.
I expect you will be leaving your charger on, in case your bilge pumps come on, if for no other reason. What does it draw?
For reference only, mine draws up to 15 amps, though likely only if the batteries are just plugged in and are low. The spec sheet gives no further information. But the caution is there, you don't want both your 600w (600w/1500w=40%) and your charger, (1500w/1500w=100%) on at the same time, or your breaker will blow and remain off until your next visit.
I my view, the charger and the bilge pumps are far more important than a heater.
I Vancouver, where my boat winters in the water, we get ice on the water every few years, as the water in Coal Harbour has a layer of fresh on top of the salt.
Since I retired, this is the first winter I have been around, so Retreat has had to survive without regular visits. My kids drop in occasionally, as do a couple of friends. I have kept this boat in that marina since I bought it, in 1994, and I know also that it was kept in Vancouver, either in a shelter at RVYC or open moorage at Mosquito creek (also fresh water on top) since new in 1980. In all that time, the most heat in the ER has been on my watch. I keep the water tank on at all times. I keep the charger on at all times. Both generate heat. The cabin floor is insulated, the fuel tanks are outside the engines, and in contact with the hull, so will be at or close to the temp of the sea water, even without the heat sources. I keep them full. After a few months without use, the temp in the ER will have dropped to ambient, but for the heat from the 2 sources I have there. I have never bothered to measure that temp, but in every case, when I have checked in the ER in the winter, it was noticeably warmer than the cabin above.
Some here have disputed the practice of leaving the hot water on. Whenever I have put my hand on the hoses and fittings on the outside of the HW, they are hot, so I know that radiant heat is coming off them and warming the ER. Likewise the charger. It may cycle, though I can't recall a time that I didn't hear it, so long as power wsa available to keep it on.
I use a heater like you have suggested, but I have it in the aft cabin, to keep mould away. I also use a lower wattage unit in the forepeak, for the same purpose. Neither of those puts out enough to keep the main cabin warm.
I have been doing safety inspections at RVYC for over 25 years. The things we Inspectors are trained to worry about begin with power cords. We check both the main supply cord and its connections and every space heater. The most frequently blamed sources of fire at marinas are electrical. Both the main power connection and all heater cords inside the boat. One thing is sure, if you don't have a heater with a large draw, you are much less likely to have a fire.
In your present situation, with only 15 amps to play with, I would leave the charge on and maybe a single incandescent light in the ER, full tanks (assuming yours are in the ER) and relax.