Yell, "Man overboard!" so that everyone on board the boat is aware of the situation, and stop the boat as soon as you realize someone has fallen off the boat.
At least one person should maintain visual contact with the victim (also called the swimmer), says the U.S. Coast Guard's Boating Safety. Turn the boat 180 degrees while having the person focused on the swimmer call out the victim's proximity to the boat.
"Litter the water" around the victim with flotation devices, says US Sailing. This will help increase the visibility of the swimmer's position as well as provide them with safety gear.
Press the "man overboard" button on the GPS chart plotter, if it's available, says Boating Safety. (If your GPS does not have this feature, you should still be able to save the location on your GPS, according to US Sailing.)

The NPS says you should not jump in after a person who's fallen overboard, as this puts another person at risk. In some cases, however, it may be necessary for someone else to go into the water. (For example, if the victim is a child or appears to be injured.) If someone needs to jump in to help, they should be wearing a life jacket. US Sailing also recommends that a rescue swimmer be tethered to the boat.