Yes if there are boats, especially with warfare, there should be pirates. A pirate is just basically a bandit or highwayman on the sea. I guess the well-known "pirate" just came to be known from the fact that they were legally licensed from their government to steal enemy boats, but "pirates" have been around far before that.
P.S. I learned in my American Government class you can still be a pirate in the U.S. if you have a license. It's kind of just a law that just never got removed. I wonder how you would apply for such a license...
I think you're thinking of a "privateer" which is basically a non-military fighting ship. For instance, when Britain and Spain were at war, Sir Francis Drake was a privateer. Not a British naval officer, but he was "legal" in the eyes of the Crown and therefore could attack Spanish ships with impunity. Technically he was supposed to stop when the war was over (otherwise he would just be a pirate) but he had become so popular he kind of got away with it.
Anyways, I support this 100%. I really wish Bethesda had spent time on this, to prove they can still push the envelope rather than just release the same game with slightly better graphics and different gameplay. Not only
would it make sense (the Nords are sailors, and even if they only have one coast, it's huge, and longboats were specifically designed to navigate inland bodies of water) but nautical adventures are at the heart of the legends and stories which inspire all fantasy. Odysseus, Beowulf, Sinbad...