I assume how 50% of the mappers don't know this:
There is a water current texture. It can be used in rivers to push players in the direction of the water. Hold on, I saw that somewhere on "The Tommy of Escondido", it is a mapper's site, which contains some interesting information.
I think the prefix for currents is '!cur_', however, not just that, you can also set it to go down, up and in certain directions: !cur90_, !cur180_ !cur270_, etc. It acts as if it's a water brush, and I don't think you need to turn it into a func_water entity. It's interesting, I tested it once, and if I remember correctly, the speed of the current is affected by how much you stretch the texture.