Quite simply, a USO is an unidentified swimming object, or an unidentified submersible object. I think most people's first reaction is to smirk and ask if the term is really serious, so I'm here to assure you that it is a real term.
USOs have been reported over the years by people in boats, as well as those who are out scuba diving below the ocean's surface. The objects typically are reported as not being organic in form, but rather that they look like vehicles of some sort, but not like anything made by humans. Naturally, skeptics have contested that most of the claims are people who have gotten worked up because they saw a sea creature or maybe even a submarine that they misidentified.
What are USOs? Nobody really knows. Some people genuinely believe that they are alien craft that might have crash-landed into the ocean, or that were built to hide in the ocean (sounds like a recent Star Trek trick, doesn't it?).
There are countless examples of USOs in movies and books. One of my most favorite comes from The Abyss. Can you name any others?
What do you think of USOs? Are they real or just people who can't figure out a fish is a fish? If you want to learn even more, check out this fairly long (45 minute) documentary on them.