A combination of cool powerboats, solid graphics, Mario Kart style racing and a sense of GTA freedom. Nice!
Then there's Powerboat GT. I wasn't expecting much from the relatively unknown game studio behind it, so the good looking graphics (especially the water) were already a pleasant surprise. The handling of the boats is nicely balanced between arcade and realism, and the overall gameplay offers some sense of freedom you find in GTA-type games. You can cruise around and accept all sorts of missions. Most of them aren't very innovative, but the weapons you can pick up in races make up for this. They are shown in boxes, like they are in games such as Mario Kart and Re-Volt and they give an interesting and often funny take on powerboat racing.
I think this has anything to offer you can expect from a boating-game, and more. The game also runs quite smoothly on a mid-spec machine with maximum detail settings.