I doubt any potential attackers are going to give you enough warning or indication of their intent ahead of time to make the standoff ability inherent in a powerful CF rifle any advantage to you as they close for their assault. Once contact is made and the attackers find themselves unable to press the attack home without resistance from you, however, they may fall back and attempt retaliation from a distance.
So you have two choices, essentially, as far as I can tell. Clear their decks promptly and completely upon first contact, or be prepared to stand them off after they open range and continue their fire once the initial assault fails. Neither one of those are attractive alternatives, but getting tossed over your own transom with your throat cut has a pretty high suckage quotient as well.
Were I planning this, every adult capable of handling one safely and effectively would have a sidearm available. In the only times I've had to carry a sidearm in a marine environment, I found a Glock quite satisfactory. It came off a lot better exposed to salt spray than the Beretta M9s the coasties we encountered a couple of times were carrying. I bought my first Glock in 1990 with the specific purpose of carrying it in a marine environment, and had ocasion to carry it several more times subsequently. It never gave me any problems.
I think you did really well finding a Winchester 1300 marine version- those are nice guns, and sadly out of production now. Learn to run it really well, and lay in a good stock of heavy loads for it- 00 or 000 buck, Brenneke rifled slugs (I like Brenneke KOs- not the sabot stuff though), maybe even some Dixie Tri-Ball (
http://www.dixieslugs.com/products.html ).
You can pretty well select your pattern size by picking different brands/loads of buckshot. A CYL or cylinder bore barrel is no drawback here. For wide open patterns, the cheap stuff (Rio, S&B etc) with unprotected soft lead pellets will get you there as a rule. The run-of-the-mill buckshot from leading manufacturers will get you mid sized patterns most of the time. And the expensive premium stuff- Federal with FliteControl wads, or Hornady TAP FPD, likewise with FliteControl, will get you amazingly tight patterns out of many barrels at pretty surprising distances. For this application, I personally would stay away from any of the reduced recoil loads. Reduced recoil means either reduced velocity or reduced payload or both, and I don't want either when it's for serious.
I don't really see any use for anything other than serious lead projectiles, personally. The "blammo ammo" stuff is a mere novelty, IMHO. If I'm going to unlimber a shotgun for serious, I want to blow big honking holes in stuff, period. YMMV of course. It's your neck after all.
This stuff will be painful to shoot in a gun that doesn't weigh about eight pounds or so, adding some weight to the gun by filling some of the stock voids with lead shot might be a help.
When I was experimenting with the then-new FN Police Pump (a 1300 clone) a couple of years ago (see
http://www.thehighroad.org/showthread.php?t=170316 ), I found recoil with heavy loads unpleasant in the relatively light weight shotgun. I added a 2-shot magazine extension for more weight up front, and filled the void under the pistol grip cap in the synthetic stock with lead shot as well. If you wanted an extended magazine on your 1300, you could always get the extension plated to match. A better recoil pad might help too.
I'd get as many 12 gauge marine type shotguns on board as there were people capable of using them safely and effectvely in the crew. Remington's 870 Marine Magnum is a bit heavier from the factory, with its all steel receiver, and they are still being made. The Mossberg 590 is made in a marine finish as well, and they are good guns. I don't have any experience with the new Charles Daly in the chrome finish, or otherwise, so I can't offer an endorsement for it yet.
I'd make sure at least one person on the crew had a full blown shotgun class, if you do decide to depend on scatterguns as primary armament. Louis Awerbuck or Randy Cain come immediately to mind as the instructors I would look for. Then that person could work with everyone else on the crew to hone their fighting shotgun skills.
JMHO, YMMV, FWIW,
lpl