Question: I am fitting a heating system to my boat and want to know which of these two systems will use less energy from my 12v batteries. System one is a heating unit which uses 190 watts at 240v through an inverter, and system two is a heating unit which uses 72 watts at 12v direct from the battery bank. Many thanks.

Answer: 
The 72W DC system will use 72 watts from your batteries, and deliver 72 watts to where the heater is sited.

The 190W AC system will use somewhere north of 190W from your batteries, maybe 220W, deliver 190W to where the heater is, and 30W to where the inverter is. If you're careful with layout, you can make the inverter deliver its waste heat to the place you want heated as well.

Either way, that's not much heat for a boat. I suggest you get a gas fired heater.
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72 watts means that the energy usage is 72 joules per second. 190 watts is 190 joules per second. Given that the "joule" is the SI unit for energy you should be able to figure which has the lower energy usage. Additionally the inverter might waste 30 watts in the voltage conversion from 12 volts to 240 volts.

So the 72 watt scenario will draw 6 amps from a 12 volt battery and the 190 watt (plus 30 watts-ish) scenario will draw about 18 or 19 amps from the 12 volt battery.