Thanks, I'm glad you took my suggestion in a positive way. It is how I meant it, but I've found it can be easy to misinterpret people's intentions sometimes in forums like this where we have many people from all over the world.
Making a water based racing game might be tricky because water is kind of linear (not a lot of vertical direction). Although you can jump boats off small waterfalls and there are wooden ramps that you could use to jump up short waterfalls.
Narrow, winding rivers could make for fun courses and you could let players set up open water courses using buoys and log booms and such.
Sprint boats, Hydroplanes, Cigarette boats, Hovercraft, Jet Skis, even Water jetpacks could all be fun or you could even invent something new.
You could even consider environments that are designed for whitewater river racing with Canoes, Kayaks or Rafts where the objective is more control and path choice oriented.