GM_Flatwater2009 by
Rubber Ducky is a nice build map designed specifically for the purpose of making water contraptions.
The big pool itself is both very deep and very long, so things like submarines and boats can fit inside it.
As seen in picture #2 above, there are little dock-like things you can plant your contraptions on. There are control panels by each dock, which when pressed, lowers the dock, so you can launch your boat/submarine.
Other handy features this map has is 41 spawn points (even though most servers don't support that many players,) it's got AI nodes so you can have NPCs on it, and more.
Download it over here:
http://www.garrysmod.org/img/?t=dll&id=63334
For more info, including support for this map, and a download mirror for this map, check out the
forum thread.[...]
