Try www.keywest.com. Mallory Square usually has one or two ships in view. In fact, if you click cruise ships on the home page, you can also find a calendar page that tells you when the various ships will be visiting. It's really interesting to watch them dock in 5 second stop-action increments. Just be watching at the right time.
Another good cam at keywest is the shipwreck tower, but the lens needs to be cleaned.