My Beneteau 36 has a 30 amp shore power connection. However, I haven't yet found a way to draw more than 15 amps without throwing the main breaker on the boat. If I am running a 1500W space heater, I can't run anything else that draws off the 110 V. Is this normal?
This type of problem is quite common with many boat owners. Your shore side power connection might be 30 amps but the problem is that your AC outlet breaker can only be 15 Amps. Most boats have all the AC outlets daisy-chained to one another. A typical ceramic heater draws around 11-12 amps at maximum draw and therefore you cannot have two heaters on one 15 Amp breaker.
The solution is simple, you could wire your boat’s starboard side AC outlets to one 15 Amp AC breaker and then wire the port side AC outlets to another 15 Amp AC breaker. This would allow you to run two heaters simultaneously and not blow either the 15 Amp AC or 30 Amp AC breakers.