When a diver jumps off a diving board, gravity pulls him or her down to the water. what is the reaction force to earths gravity?

The reaction force to earth's gravity on the diver is the upward thrusting force of water.  

Explanation

The water present in the pool will be exerting an upward thrusting force on the diver which will reduce the speed of the diver first and then make the diver to float in water.

Thus the diver won't be going directly to the ground as the gravitational pull on the diver will be overcome by the upward thrusting force of water on the diver.

On the diver, this upward thrusting force will be acting as the reaction force to the gravitational action force.