I have a M37 from 1939. It's seen lots of use over the years but is absolutely the smoothest cycling Shotty I've ever used. The barrel is trimmed to 18.5".
Get a RICOL "14" Short Over and Under with Screw in Chokes from corwin arms (site sponsor here) then order a .22 LR insert for 12 gauge from the state, problem solved. You can have a 12 gauge and .22 LR in the same gun. Go to the range and learn how to shoot those .22 LR insert accurately, that would be my suggestion.
Usually, I pack two inserts when I'm going to the bush. .22 LR and .45 ACP.

Remmy 870 with 18.5" - 20" barrel. Mossberg 590 with similar setup would also be excellent. Nothing wrong with a Marlin guide gun appropriately loaded as well.
Wow I'm surprised no one mentioned the AR7 yet. They're great for canoe trips, they pack down to fit in a small pack and they float
Not so good on bears though
We should keep a sound perspective on shell size sub-calibre inserts here.
This is my singular experience with 38 Special American Eagle 158 gr LRN to 12 gauge shell size inserts (yes some rifling) from an Ithaca Deerslayer at 17 yards from a bench.
BTW, at least two of these shots were taken at 10 yards, but performance was equally dismal and I forgot which of them are this pair of closer shots.
Not really inspiring IMO.
Short barrel 870 with butt-stock shell holder. The bandoleer is a wall hanger, never take it with me anywhere, but it looks neat. The shell holder load out has settled on a couple of slugs, a couple of double-0, a bear banger (goes off at 50 yard stand off) and a flare. I figure if anything ever saves me if I get into trouble, it'll be the flare, after I fall and twist up my ankle, knee, or hip, and can't walk out of wherever I've gotten myself up to. Need to be cylinder bore to fire flares, or else they can get stuck.
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If you're canoeing, no reason not to throw in a Papoose as well. Handy little rifle, more accurate than my 10/22, holds zero between removing and re-attaching the the barrel. Really quite fond of this little rifle, and they go for around $180.
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