Heating water with electricity on a boat is a daunting prospect...
A Joule is an energy measurement. Basically 1W per second
It takes 4 Joules to raise the temp of 1ml of water by 1DegC to do that in 1 second would require 4 watts.
Take the kettle boiler linked for example - 20Oz = 600ml
Room temperature water is about 20Degc so I need to raise it by 80 Degc
4J X 600ml X 80Degc = 192000J
The unit is 8 amps = 12 X 8 = 96W
192000 / 96W = 2000 Seconds or about 33 minutes to boil. Even a big cup of coffee (300 ml) would take 15 minutes
It would also take about 4.5 amps. To get the water boiled in say 3 minutes (like your kettle at home) you'd need a unit that does about 80 amps or 960W.