One of my assignments at Uni was to create a 2D boat racing game that allowed at least 2 players to play over a network. Well, now that I've finished Uni I'm making the game available for you to download and play.
Its coded in Java, using Eclipse IDE.
Max players is 200, but 4 is recommended.
There's no anti-hack stuff, so only play with people you can trust.
If you want to host a server make sure UDP port 1337 is forwarded to your PC.
Its pretty easy to mod the maps, add new ones, add new boats, just open the .jar with WinRAR or something and go to boatRace\assets.
All boats have the same stats apart from the jet ski (slower, but higher acceleration) and wooden raft (faster, but slower acceleration).
Move using WASD or arrow keys
Hold shift to boost
Mouse wheel to zoom in and out
Esc to exit
You will need at least Java 6 or 7 for the game to work (can't remember which).DOWNLOADSOURCE DOWNLOAD