In Residue processing, also while going through the tunnel in the dam is another time I can think of such a time it occurs. But on Residue Processing I rememeber a part where you fall down off a conveyor into some water with 2 doors that are locked and the only way through is by swimming. Here let me find a video with the area I am trying to talk about.
Here we go, skip to 8:20 into the video and thats the place I am refering to.
https://www.youtube.com/watch?v=UI3wbAKTJF4Maybe it was just that first part that pushes you, or rather supposed to.. It seems like it should work by doesn't now. But, I'm quite sure the dam did push you, unless that's broken as well.
Also thanks for the ideas I will try the func_conveyor, though I don't seem to ever have much luck with those when I make them not solid. Also I found someone else who said to "Use a func_illusionary with water content - paint the illusionary the color you want the under water effect to be, then stick trigger_push inside." So I will try that to see if that works.