In our last two boats we have had the Dickinson propane fired fireplace cabin heater. While not as efficient as the espar types, it is much more trouble-free. This particular model (P-900 or P-1200 - the number is BTU output) has a dual flue, pulls combustion air from outside and vents flue gases outside as well so there is no condensation issue whatsoever. There is a built in fan that, though noisy at high speed, greatly enhances the effectiveness of the heater.
Another plus is that the flame is visible throught the sealing front door, giving a very pleasant ambience in the cabin as well as the warmth. We have the p-900 on a 34 footer - but might have chosen the 1200 btu model in hindsight.
here's the link to this heater. It's clean, its safe and it's compact and relatively inexpensive.
After looking at dozens of used boats over the last few years, while many had espars installed, most seemed to be out of commission.