Your map apparently leaks. Check the compilation log - it should say:

-- leaked --
Entity entity_name (x_pos y_pos z_pos) leaked!

yohoat9 wrote:
But what do I do to fix it?

This happens every once in a while with water for me; the next time I compile, it'll be gone. Very odd, and I'm guessing it's just a bug. If there is that flag in the compile log, then there's an entity that's outside of your map (e.g. projecting into or touching the "void"). Under one of the drop-down menus at the top, click "Load Pointfile". If there's a leak, a dialog will pop up saying "Load default pointfile (.lin)?" Hit yes. Now, there will be a red line in all four views that stem from wherever the leak is coming from; fix it so nothing's touching/protruding into the void, hit "Unload Pointfile" (under drop-down menu again), and compile. That should fix your problem.

If your map doesn't have a leak (anymore) and the problem still persists (i.e. it's not a compiling freak accident like WinstonSmith described), then there is probably something wrong with your water volume. Check that:
- the water volume is flush with the walls on all sides and the bottom of the pool,- the water texture is only applied to the top of the brush, all other sides must be "tools/toolsnodraw".
EDIT: some more info on making water:
http://developer.valvesoftware.com/wiki/Adding_Water