Some of the referee signals in beach volley ball are surf, where the referee tells you it's time to surf. He'll blow the whistle and make this motion.
If the referee calls the ball in, it's with his arm down and out with the arm up. If there's a lines person they might have a flag and they have the flag down for in and the flag up for out.
If there's a touch on the ball the referee will go like this to indicate that the player has touched the ball. If someone doubles the ball the referee will hold up the sign for double.
If the player touches the net the referee will make this motion towards the net. If a player lifts the ball on a defensive contact the referee will make the lifting motions.
And those are the basic referee signals in beach volley ball.