There are already a few models you can use. Mannhattan has a skybox cargo ship model that you can find in Hammer, the Frontline pack has a tugboat and a landing craft with skybox versions, and there might be a few more options in the game files and/or in some unofficial prop pack. Alternatively, you could just make your own with brushes, as Observer suggested. They don't have to be that great or detailed if they're only gonna be in the skybox.
But, if you're picky and/or really do have a specific design for a model in mind, you're going to need to be a lot more specific. There are tons of variations among boats and many, many ways to design a boat. At a very minimum, you at least need to say what type of boat you want.