If you like Hydro Thunder GO! but not the price or think the physics could be better, take a look at 3D Boat Race from A Trillion Games (makers of 3D Paperball). It's your typical boat racing game but (1) it's free (2) has some really nice physics and gameplay to it:
"Paint your powerful speedboat and challenge other Windows Phone users on 100 different tracks, to become the top pilot worldwide! 3D game with realistic 3d physics, smooth rendering, fantastic gameplay and real-time global leaderboards. Hours and hours of fun! "
At only 6MB in size it's pretty damn impressive and certainly a good deal for being free. Just check some of the action in the above video to get an idea. With that, we have to give a thumbs up for you to at least give it a try, after all you have nothing to lose.
Pick up 3D Boat Race here in the Marketplace.