See Ordnett.no for these definitions of the English "fin":

fin subst.
1
(zoologi) finne
2
(luftfart) siderorsstabilisator, halefinne, styrefinne

It's common to adopt marine zoology terms for boat parts, especially paddlecraft such as the kayaks sold by the source of your source.

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Note added at 12 mins (2010-08-17 12:28:30 GMT)
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You can see an example of a fin for kayaks at http://www.austinkayak.com/products/40/Wilderness-Systems-Ka...

(You might see "skeg" as well, but that's generally used to describe a substitute for a rudder, not a fin such as your source employs in its boats.)

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Note added at 17 mins (2010-08-17 12:34:10 GMT)
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My discussion of skegs was intended to be limited to the kayak realm. The word has a different meaning on larger vessels. Here's a good description of a kayak skeg and its operation: http://www.atlantickayaktours.com/Pages/ExpertCenter/equipme...

Okay, I promise to stop talking about kayaks now. I sold them for several years -- hard to break some habits . . .