Mike,
Propane heaters are dangerous around gas tanks because they are a potential source of ignition. Any gas fumes and you're in big trouble. Same reason why you turn the bilge blower on for a while before starting any inboard or I/O. The other significant danger in boats is that propane sinks in air so if you have a leak it ends up in the bilge. Having said that...I run a heater just like Bob describes in my driftboat but be very careful if you put one in a power boat with enclosed bilge. I think you want to mount the propane tank above decks so you can smell a leak before it goes too long and of course keep the burner away from anywhere that could have gas fumes. I have seen this done on some of the high end salmon boats (Skagit or Osprey?) so it is possible to do safely. Hope this helps.
JAM