Always begin with buoyancy you reduce the immediate risk by assuring that neither Open Water Diver Manual

Of you will sink. To do this, ideally throw or extend some flotation to the diver, but if you can't do that, inflate the diver's BCD and/or discard the weights. Once you've established buoyancy, calm the individual by talking, offering encouragement and persuading the diver to relax and take it easy.

Have the diver take deep, slow breaths to reestablish breathing control, and self-control. After sometime to rest and recover, if necessary assist the diver using the tank valve tow or the modified tired-swimmer carry, which you'll practice during Confined Water Dive Three.