Yes, Pilou's tip is good - extrude using the "Set path" is one good way to make a wave.

There are a few other methods you could use too - you could make a flat plane made up of many control points and then pull surface points up or down, see this post for an example of that method: http://moi3d.com/forum/index.php?webtag=MOI&msg=863.2

Also a one-rail sweep can do it. For example, after drawing these 2 curves:



Select one of them, then run Construct / Sweep, pick the other curve as the rail and you can construct this swept surface:



You can adjust the shape of the profile or the rail to change the shape, and you can also use more than one profile if you want to gain more fine tuned control.

Make sure you build your wave to be a little larger than your glass, then you can use Edit/Trim or a boolean to cut the wave surface by the glass to remove the outside part of the wave.

- Michael