I know how to make water, but how do i make the water so that it has the underwater sound and the underwater effect when your in it?

You should be able to hear it, but maybe they removed the sound in Garry’s Mod, cause it’s rather annoying most of the times.

I only have played the map after I compiled it so I don’t know what its like in garrysmod.

Turn up your volume.

A nodraw brush with a water texture on the top should have proper effects and sounds by default.