Ducks and polar bears are two different problems, requiring two different guns. Your bear gun should have rifle sights rather than a bead. IMHO, a ghost ring and rectangular post trumps all other types. If you intend to do any exploring on foot while on your adventure, a sling and swivels will be helpful. A side saddle is a good option should your intend to use cracker shells and rubber slugs in addition to lethal ammo. The best way is to have the magazine full of slugs, then single load the less lethal rounds as required. That way if the situation deteriorates, you have instant access to a slug with no fumbling.
If traveling with a companion, one could have a scare gun and the other the kill gun, don't mix em up! Cracker shells are corrosive, so be sure to clean the bore with hot soapy water as soon as possible after shooting them. Another option is to carry a scare pistol which illuminates the need for mixing ammo in your shotgun.
The brass on shotgun shells corrodes quickly when exposed to the elements, and for that reason I would choose a gun with a tube magazine that has a spring and follower that is easily removable for maintenance. A steel follower that rusts fast to the inside of the magazine tube will spoil your day. The gun is going to get wet, and if its going to function when you need it to, you'll have to look after it. Too many people purchase the cheapest guns they can find, and then treat them like clubs. Shotguns are far less robust than their rifle counterparts. Barrels and magazines dent easily due to their thin wall construction, and a dented magazine tube reduces your repeater to a single shot.
As for the make of gun you should choose, my first choice is a Mossberg 590 (which is what I carry when I choose a shotgun rather than a rifle) and my second is an 870 Marine Magnum (which is what the DNR guys carry as do Parks Canada's people, Churchill Northern Study Center's staff and some of the buggy operators). The parkerized finish on my 590 offers good protection from the elements (a 590 with a marine finish is available) but without attention, the 870 Marine Magnum will stand up to the use of corrosive ammo longer than a parkerized 590.
After your trip, buy a nice 20 ga for ducks.
With respect to less lethal ammo in general, and cracker shells in particular, practice with them before you need them. If you drop a cracker shell on the far side of the bear, you've just arranged for a train wreck. Should you have to use lethal ammo, to save yourself or a companion, don't shoot for the chest. The point of shooting is to stop the bear and a chest shot won't do it quickly enough. A brain shot works, but is difficult because the bear's head is very wide, and unless the bear is focused on you, its in constant motion. Additionally, the brain is only the width of the snout, and the bear's forehead is low. A spine shot might be a better choice. If the bear is after your companion, attempt to break a large supporting bone like the hip which should be the farthest viable target away from your companion, to stop his forward progression. You then have time to reposition to make a kill shot.
If this is your first gun, I then assume, rightly or wrongly, that you have little shooting experience. On the job training is a difficult way to go when it comes to personal defense, and I would like to see you put 500 rounds down range before you needed that gun to save you. Concentrate on shooting at close range, a bear that can't touch you can't hurt you, so by definition defensive shooting is a close range gig. Practice snap shooting 10 yards and in, practice loading and unloading, practice shell select, whereby you load a type of ammo other than a slug, fire that round, then repeat without getting a slug from the magazine in the chamber. And get used to stripping that gun and putting it back together until you are comfortable with the process.
A word about slugs. Get Brennekes. If you can't find Brennekes, get Challengers. Use foster slugs only as a last resort, they're too softy, expand too quickly, and lack penetration.
Recovered slugs, Brennekes compared to a Foster slug
A Challenger Slug
Aside from the gun question, be sure to print off and laminate a Tide Chart for the Nelson/Hayes River part of the Hudson Bay coast. You could be in salt water 10 miles before you get to the coast, depending on the height of the tide when you get there. The tidal flats on that part of the coast extend for miles into the Bay, so don't get caught out there when the tide changes.
http://www.waterlevels.gc.ca/eng/data/table/2013/wlev_sec/4980