and they also make a rifle version, in .223 Remington/5.56x45mm, that is VERY well regarded. It takes AR-15 magazines (common, inexpensive, and reliable) and stores quite compactly, and gives you better ballistics than a pistol caliber carbine while retaining the advantage of light recoil.
Here's the Kel-Tec pistol-caliber carbine:
And two versions of their folding rifle:
SU-16A (California legal)
SU-16C
The only concerns I've ever heard about the Kel-Tec carbines is their relative fragility compared to some other carbines, due to the lightweight construction and the way the folding mechanism is configured, and the fact that they are rather prone to rust if not kept well oiled.
A stainless Ruger mini-14 or Mini Thirty with a folding stock and good magazines could also be a decent choice. Mini's are absolutely reliable, are easy to take apart and clean without tools, and aren't fragile. Here's what a mini-14 looks like with a folder (this is my mini-14 Ranch Rifle with a Butler Creek stock):
Stock extended
Stock folded for storage, with 30-round magazine.
That barrel is 18.5", but you could have a gunsmith cut it down to 16.1" and move the front sight back, and it'd be even more compact. It's not the most accurate gun in the world (5" groups or so at 100 yards), but if you're shooting from a boat, then reliability, capacity, and good ergonomics are more important that the ability to shoot super-tight groups on dry land. Also, if you put into a port where the local authorities are squeamish about modern-looking rifles, you can make the mini look like an old-fashioned cowboy-style carbine in about a minute, by swapping the stock:
With the straight stock, the mini-14 is also legal in California, where protruding handgrips on rifles are against the law. (Note that the Kel-Tec SU-16A rifle is California-legal, whereas the Kel-Tec pistol-caliber carbine and the SU-16C are not.)