The answer is on this National Geographic page about the best pictures from NASA's official photographer Bill Ingalls:
If you love space, odds are you’ve admired the work of Bill Ingalls.
He has been NASA’s senior contract photographer for 30 years, a job
that has taken him across the world—but not yet beyond it—to cover
major moments in space exploration.
His stash includes two Nikonos underwater cameras, which, Taub told
Ingalls, "were used by frogmen during Apollo splashdown recoveries."
In this DOD Pdf about Apollo Recovery Operational Procedures equipment furnished by NASA is mentioned. Underwater cameras, 1 or 2 per recovery diver team and 3 per primary recovery ship.
Nikonos model II image from this Nikonos Story page.
The size of the camera was: 129 mm × 99 mm × 47 mm (5.1 in × 3.9 in × 1.9 in), the weight 495 g (17.5 oz). The Nikonos was marketed as an all-weather camera. Photos below and above the water's surface are possible, also in heavy rain.
An Apollo 12 image ap12-S69-22265 from ALSJ:
A Navy diver helps Al Bean into the recovery raft. Pete Conrad is at
the far right and Dick Gordon is at the center. Note the diver in the
water to the left of the raft taking pictures. Photographs are also
being taken from the helicopter. 24 November 1969. Scan by Ed
So at least two divers were equipped with Nikonos cameras, the diver shown with camera and the diver who has taken the image.
Taking pictures by Navy divers during recovery has a tradition:
Navy divers prepare to retrieve the Gemini 6A crew on16 December 1965.
Green dye was released by the spacecraft on splashdown, making it
easier to spot them from the air. Credit: Courtesy of NASA
Image and text from amateurphotographer.
But the Nikonos I was introduced in 1963, so it might have been used only for the last Mercury flight.
Bold font enhancement within block quotes by me.