The Maruzen M870 works fine on propane (mine does anyhow..). Im not sure I would want to see it on duster with any more than 3 BBs though.
Mine is also full metal and is a BEAST, I havnt held a Tanaka 870 yet but I would bet mine would top it on weight and feel. Another plus of the maruzen is the cost of shells is much lower, so when you lose them (will happen) it doesnt hurt as much. For use of the ferry it shouldnt be terrible, I've used both my M1100 and M870 on panther and wile it left something to be desired I was still able to have fun and get kills.
If your thinking of getting the maruzen brand brass catchers you may want to see if you can find something else... they suck. I dont bother using them anymore, just try and grab the shells as I go. On the ferry that might not be such a great idea, espically on the open deck.
Another thing to note about the Airsoft Surgon slug shots is that they dont use BBs! Watch the vids, they use rubber plugs (hence being a slug shot). I dont know what they charge for replacement shots but either way, that sucks, might as well get the CA870.
The M870 may be weak (the M1100 is more powerfull but significantly less reliable) but can be upgraded (CO2 mods) to shoot a respectable load of BBs. On the other hand the Tanakas come out of the box ready to rock but with expensive shells and a max of 3bbs per shot. Really up to personal preference. Either way airsoft shotguns were described to me as a fine wine, to be taken out on special occasions and enjoyed.
On a side note, if anyone knows how to get ahold of extra Maruzen M870 loading ramps I could REALLY use a few extras...