only if glock made a shotgun that would be your best bet lol
Not to stir a hornets net but I've had multiple glocks- both broke at the range...
BPS All Weather is a good gun:
http://www.morrisonarms.com/2012/04/browning-bps-all-weather-20-stainless-barrel/
In my opinion, too big for a boat gun, that needs to be a takedown. It should be 12 gauge, to be able to shoot flares, birdshot, slugs & buck with equal aplomb. It should be a takedown, for the boat space, especially in small craft (very much so in a kayak). That brings me to doubles.
The ultimate, no expense spared boat gun would be a side by side, stainless 12 gauge (doesn't exist), side by side as the shallower opening angle will be easier in a boat. 18" barrels, the whole package would be a foot and a half long, and side by's are known for their reliability (reference African dangerous game doubles).
The discontinued Ruger Red Label All Weather O/U shortened up would get you closer, but not there.
Absolutely agree on the 12g for all the reasons you mention,I don't need a take down, it will stow in a scabbard and needs to be available at a moments notice.
The BPS has my attention as parts of it are stainless and I had a BPS bird gun before and it was a very refined shotgun. The sight I'm not so sure of- it looks like something could get hung up and snagg, or break. I'd really like to handle one before I spend...
The answer could be very different depending on where you intend to sail, the west side of Hudson Bay presents different challenges than say the coast of Somalia. But where the problem is best answered by a 12 ga repeater, my life experience suggests that the marine coat Mossberg 590 would best fill the bill. I've seen enough problems with the Remington Marine Magnums in the last week to turn me off for life . . . five out of six that I examined had cracked or broken ejector housings, the ejector housing has a finger that indexes the barrel position, so once gone, only the barrel band prevents the barrel from rotating left and right. By comparison, the barrel lug on the Mossberg indexes with a cutout in the receiver, and is comparably very robust. Over time you might find that the shell stops on the 870 come loose and must be re-stabbed or the action will tie up, whereas the Mossberg's are designed to sit in their recesses and are held in place by the trigger group. Finally, the ejector on the Mossberg is solid and held in place by a screw with no spring, and can be changed out with only a simple screw driver.
Gold! thanks, narrows one out of the running

With the 590 mariner; does the mag tube spring and plug come out the front (cleanout) or is it like the 500 (bolted- must remove the mag tube to clean it) ?
I have a 500 now and while it has seen a whack of abuse there are things I just don't like- they barrel retention bolt is one, and the cheap plastic forestock is the other (does the 590 use the older metal sleeve under the forestock?)
Seems like the mariners I've seen stocked or advertized have 9 shot mag tubes and fancy sights & stocks- This is not what I need, a shorter mag with a plain bead (a tritium would be a plus but I haven't seen anything like that) and perhaps a speed feed stock - or plain.
Also, is the 590 an aluminum receiver or steel?
Also, doe it use chokes (I would consider this a bad thing)?
It would be so much nicer to have a shop that stocked these things so a guy could get to touch them before dropping cash...
Thanks
Oh, and an Iowa class drafts to much water, the Dillon Aero looks like it could work- do they come in stainless,lol...