The agents, members of the CBP Office of Air and Marine, spotted the panga early Thursday morning near Encinitas, hailed the occupants and ordered its pilot to yield.
The motorboat continued on, prompting the government agents to fire warning shots, CBP spokeswoman Jackie Wasiluk said. The two vessels then collided, causing the skiff to capsize and sending its 20 occupants into the ocean.
The agents pulled the group out of the water, then performed CPR on an unconscious woman, Wasiluk said. A U.S. Coast Guard helicopter crew airlifted her to a medical facility, where she was pronounced dead.
The cause of the unidentified woman's death was not immediately clear.
The other migrants were taken to shore. Medics took four of them to a hospital for treatment of apparently non-life-threatening injuries and tended to the remaining 15 at the scene.
The entire group was believed to have entered the United States illegally, the spokeswoman said. It was not immediately clear if the suspects might have been transporting any type of contraband.