New Jersey State Police made the call for help to the Coast Guard at about 1:35 a.m.
A rescue helicopter arrived from the Coast Guard Air Station in Atlantic City, N.J., and found a boat taking on water.
Boat crews from Coast Guard Stations Cape May and Indian River Inlet, Del., also arrived on scene. They found both men.
One of them was responsive and taken aboard a Coast Guard boat, then flown for treatment. He's listed in critical condition.
The other man slipped under the water and has not been seen since.
The Coast Guard is still searching the area.