I have to admit, that my cross training program involves playing tennis, purely because I like to play tennis. However, I do think the two sports are very compatible. The service action in tennis is quite simillar to voleyball, and the basic principals of movement and ball control are basically the same. In both sports, good footwork will allow you to produce a better arm movement.
The fitness aspect of tennis is very simillar as well. Short bursts of action, then a breif pause between points. One important lessaon you can learn from tennis is to back up from a very tough point with a solid serve. It is very easy to win a huge point, go back to serve and hit a double fault because you’re still puffing.
In beach volleyball, maintaining consistency, especially just after a big point can make a huge difference. When your opposition is out of breath, and a bit tired, you have the best chance to make a couple of quick points, which can often be the difference between winning and losing. A lot of this comes down to the service routine I have talked about previously. If you watch a professional tennis player, every time they serve the ball, they do exactly the same thing leading up to the serve. I suspect that they think exactly the same thing too.