I think it's time to review some facts about electric
heaters. All electric
heaters are 100% efficient. That is, ALL of the electricity consumed is converted to heat. Heat is the movement of air molecules. Electric heat does not come in "wet" or "dry" flavors. It's just heat. So buy a $1,500 imitation Amish fireplace, a $50 ceramic heater, or a $14.96 Wal-Mart box heater. They'll all put out the same amount of heat, for the same cost in electricity. The oil
filled heater, as Dockhead points out, is just doing an extra step of transferring the heat to a liquid, so it radiates out into the room more slowly. They're also quieter, since they have no fan.
An average US household AC outlet is rated for 15 Amps. Watts = Amps x Volts. 120 x 15 = 1,800. You don't want to max the circuit out with just one appliance, so you'll find that virtually all plug-in electric heaters produce 1,500 Watts on "high".
If you have 30A service
to your boat, you'll only be able to run two of these. 50A and you should be able to run three. In either case, they must be on separate circuit breakers.
For reference, 1,000 Watts = 3,413 BTUs. 1.5 x 3413 = 5,119.5 BTUs from one heater, or just over 10,000 BTU running two.
There are lots of factors to consider when calculating how many BTUs you need. On most cruising-size boats, you won't get by in real winter weather
with only 10,000 BTU. Maybe in a mild climate; you'd have to check the figures for your location.