If it will help figure a proper sized heater it may help to know that a 1500 watt electric
heater is just a fuzz above 5000 BTU's. You were running two so you had around 10,000 BTU's and that wasn't keeping your boat warm. Using the approximation where you take the length, width and height of your heated area and multiply by 14 to get an approximate BTU required.
If your heated area were 35 feet long, approx. average 10' wide and 6.5 feet high multiplied by 14 that would seem to indicate about 25000 BTU. That's the equivalent of 5 of your 1500 watt heaters. Sound about right?