I've had one and they are generally an "at the slip" option. when under sail or in any kind of wind (read > 5 knots) these things will not even boil water. they are fine grills but I would recommend dummy cording the hood to the unit as i found that it tends to want to sacrifice itself to Posieden.
As for the fire hazard with things nearby I'd recommend that as long as you keep the stuff upwind of the grill you should be fine. If you can't insure it will be up wind then a 2'clearance will probably be OK.
the newer grills, like the magma marine kettle II, has the lid attached on hinge points...a great improvement. and it will definitely boil water in a wind storm.... and quickly!
the older ones can be upgraded to the newer style, but for the cost, a new grill may be the way to go if that is what you want
and in addition to the "questionable" heat output, magma offers three different options for regulators, depending on your style of cooking... Low output, standard, and High output...
as for mounting a grill on a boat, it should be mounted outboard of the railing.... so any grease drips or food that may catch on fire can be scraped off into the bay and not into the boat.
and that way it shouldnt interfere with the rail seats.