We tend to paddle with the same people most of the time, and during that time we have come up with a whole scala of river signals. Most of them require little imagination, and besides the first couple of basic signals, they are only used in situations where time is not very essential (reuniting swimmers on one shore with their boat on the other shore etc.) It's just comfortable to know more than the basic signals when you want to communicate over the roar of water in between.

We tend to use the "hand over eyes" signal to indicate that you need to look at something, for instance a dangerous spot, a possible line or just something funny. This tends to prevent confusion about whether or not pointing at something means "go there" or "stay away from that". This list is just a selection of what we use, and far from complete. I know of some paddlers who actually comprehend sign language (for deaf people), and they can keep entire conversations in detail. For us, this list works well.

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Wilko

Go! Also known used for "Come on down to this eddy". Very practical if you can't see the next eddy or the paddler in front of you. The lead paddler can signal above a rock or some other obstruction that blocks sight.
Stop! You're expected to head for the nearest eddy a.s.a.p.
Go on that side of the rapid! (pointing either left or right). When you've been paddling together for some time, the angle of the paddle can mean more or less in that direction. We usually extend the entire paddle in a pumping movement that direction to indicate "Go further right/left!"
Strainer! Crossing your arms above your head. This also means "danger ahead", so catch an eddy right away! We paddle in an area where farmers sometimes put barbed wire across the water, and often the flooded river finds its way through meadows, going under barbed wire.
Drop! Moving the hand like diving off something.
Drop with (shallow) landing on rocks! Beware!
Portage! (or "someone's walking") Might also be useful to indicate where a swimmer is walking to get an effort going to unite him/her with paddle and kayak.
Look! Indian style "hand over eyes" signal, often combined with an extended arm pointing at what needs to be seen.
Faster! Making paddling movements in front of your chest.
Swimmer! Making swimming movements in front of your chest, accompanied by pointing at the swimmer.