Finally found a fun boat racing game after playing random free roam games like GTA to get my boating fix. Its called aquadelic GT and its only $5 on steam. Not much about it is realistic but the idea is that you can roam freely around a bunch of different large maps that are littered with small missions, races, boat shops, and houses that you can buy. Think of it as Test Drive Unlimited for boating.
There are a bunch of boats available ranging from a small runabout to twin engine speedboats. You have to complete races to advance to new locations which have new boats and other events. Most of the races are an appropriate difficulty and are neither easy nor frustratingly impossible. I already have the most expensive boat in the game after about 2 days of playing but I still find it entertaining to cruise around the various maps or just do single races for money.
Id say its pretty much essential to have a gamepad to have any fun, and a wheel would be a blast, but I havent tried with the keyboard so im not sure. Races are kart style and have boost and weapon pickups that seem well balanced except for maybe the huge jumping heat seeking explosive frogs...
Graphics are nothing special but the game runs well and the water effects are actually pretty well done. Here is an ingame race screen from early in the game when I was sponsored by a bubble gum company:
In conclusion: If you like boats and having fun, take a look at this game.