Special effects artist Chris Corbould has worked on creating and developing all the vehicles used in Christopher Nolan's Batman films, like the Tumbler, the Batpod, and The Bat. In an interview with Digital Spy he reveals that he was pushing for a Batboat to be in The Dark Knight Rises, but obviously Nolan passed on it.
I was heavily pushing for a Batboat, I love working on water and thought it would have been a great Bat-vehicle. Chris felt that it wasn't big enough for the last in his trilogy so he went for [The Bat instead].
I would have loved to see a Batboat, but I'm happy they went with the flying Bat vehicle. It was pretty damn cool, and I don't know how the movie would have played out with a boat. Batman wouldn't have been able to pull off most of the stuff he did in the film without it. I would love to see the designs of the Batboat though if they exist!
Watch the video interview below in which he also talks in detail about The Tumbler.