Always bring lights — that's one lesson Nick Caloyianis learned from decades of filming and photographing underwater life in the Chesapeake Bay. His photographs document the loss of natural light in the estuary. In 2008 (above top) he worked with multiple lights as he tried to film oyster restoration on Dominion Reef in the mainstem Bay. Back in 1979 (above bottom), he had plenty of light to photograph his partner, Clarita Berger, as she took pictures of the grassbeds of the Choptank River. Credits: Michael Eversmier, Aqua Ventures, inc. (top) and Nick Caloyianis (bottom).

IT LOOKS LIKE OUR LAST, BEST CHANCE FOR THE YEAR. Motoring out of Cambridge marina, we can see some skim ice floating below the docks, but all the signs say this is a good day for diving and filming and photographing the bottom of Chesapeake Bay.

We had the right weather: windless, clear and cloudless. We had the right season: two days before Christmas, two days into winter when cold waters can run clear, with no warm-weather plankton blooms to block out sunlight.

And we had the right cameraman: Nick Caloyianis, a Marylander who has done deep-water filming all over the world. Lean and lightly bearded, Caloyianis and his partner Clarita Berger came well-equipped for a day of shallow-water diving in the Choptank. They've loaded the cockpit of a wide-beamed workboat with crates and piles of scuba gear and camera gear and underwater lights.

As we cruise the middle of the Choptank River, the sun — still low in the east — begins bouncing light up off the river. Behind us, it slowly lights up the brick and frame houses that stand like sentinels along the shore and lends a soft glow to all the white sailboats and cabin cruisers that now crowd the waterfront. Would there be light below the water? That's the gift we're looking for. But the river gives no clue. We are cutting across a flat, flashing mirror.

As a filmmaker, I first hired Nick more than 20 years ago to dive down and film the Bay's dwindling seagrass beds for a documentary called Chesapeake: The Twilight Estuary. Since then Nick has filmed all around the world, from the waters off Galapagos and the Greek Islands to the Red Sea, the North Atlantic, the Arctic, the Caribbean, and the far western Pacific. In recent years, however, he has refocused his energy on his home waters and refocused his career on a new role: he's trying to turn himself into an unusual kind of naturalist, an underwater naturalist.

According to the classic definition of the species, a naturalist uses direct observation to study plants and animals in their environment, in most cases bringing back data in the form of field notes and samples for lab analysis. It's a definition that fits scientists who focus more on field work than laboratory experiments, molecular biology, or theoretical modeling. And it's a definition that fits certain nonscientists, including writers like the famous Henry David Thoreau and the less famous Gilbert Klingel. Back in 1951, Klingel wrote The Bay, one of the first popular books on the Chesapeake. Klingel, it turns out, is an inspiration Caloyianis likes to cite, not just for his book, but for his early interest in directly observing the underwater life of the estuary. He spent time on the bottom in old-style diving suits with metal helmets and in a diving bell he invented himself. He brought back some of the first photographs of the underwater Chesapeake.

The naturalist label, by these definitions, may also fit a photographer like Caloyianis. For a number of years he's been trekking out with all his scuba and camera gear to film and photograph the key species in the Chesapeake, both the famous and the little known: not just blue crabs and oysters and striped bass, but red sponges and toadfish and killifish, all shown in the underwater world where they live, a world the rest of us — including most scientists — never see. His new goal is a book he's titled Life Beneath the Chesapeake. He sees it as "a big picture look" at the underwater estuary: full of color photos that would give — species by species — a view of the whole system. He's after "the quintessential elements that made the Chesapeake what it is."

If Caloyianis can pull it off, his picture book could make the perfect gift for some future Christmas — but that's a big if. He's got better gear than Klingel ever had, but he's got a bigger problem to solve. The key to great photography is light, and light is hard to come by in the Chesapeake, much harder than it was when Klingel made dives in the mid-1950s or when Caloyianis himself first began diving and photographing in the mid-1970s. As a beginning photographer who couldn't land many assignments, he worked for two years as a commercial oyster diver. "I didn't make that much money," he said, "but I saw a whole lot. And every chance at good visibility I took my camera in the water." Down along the bottom of this same Choptank River he made pictures showing 20 to 25 feet of visibility. "Look how clear the water was," Caloyianis said. "We haven't seen that in a quarter of a century."

Today he's back on the Choptank to film the bottom for another documentary, and he'd be happy with just four to six feet of visibility. Even that kind of clarity is rare, he warns, on the order of one day in ten. Some days he can't see his hand in front of his face. On those days he packs up his gear and goes home.

Caloyianis, however, has learned a lot of tricks over the years. The key trick is persistence, to keep hauling his hopes and his gear down to the Bay despite being skunked. Another trick is to always bring lights. The best tricks are to pick his spots — two favorites are Eastern Bay and Mobjack Bay — and pick his days. He checks the marine weather reports, especially the wind reports, and pulls up the latest satellite imagery that can show an educated eye a lot about current turbidity patterns. He also calls John Volatile, an ex-waterman who keeps a constant weather eye on the Bay. Lately Caloyianis has gotten his batting average up to six days out of ten.

We're still hoping for a 10-foot day when Ben Parks, our captain, drops anchor out in the middle of the Choptank, just east of the Route 50 bridge. Parks is a long-time waterman with local knowledge and a GPS. Both tell him we're floating directly above Bolingbroke Reef where seed oysters were planted three years ago. Back in the cockpit, Don Meritt, a scientist from a hatchery at the UMCES Horn Point Laboratory, swings a set of long-handled tongs over the side and starts working the bottom. When he pulls up a load of big oysters, we know we're on target. The oysters are clumped together and clotted with mussels, another good sign. Oysters and mussels are strong filter feeders and restoring them to the bottom could help clear the Bay's murky waters.

While Caloyianis swings his oxygen tanks across his back, Clarita Berger leans overboard and tries her spit test. Her small glob of spit slides quickly past the boat. A strong tide is running — not a good sign. Both divers gear up, buckle on their weight belts and slide overboard. Captain Parks hands a heavy underwater camera down to Nick, and the two divers sink out of sight, swimming hard against the current.

When they find the reef, it's mostly by feel. Down along the bottom, the divers are not getting a 10-foot day or a 4-foot day or a 2-foot day. Visibility is down to inches and the two divers have to hang onto each other to keep in sight and hang onto rocks to keep from being swept off the oyster bed. Caloyianis surfaces with several minutes of footage, but it's too murky to be useable, and he doesn't ask for his still camera. Healthy oysters are down there but not enough of them to clear the water. Despite all the planning, he's been skunked again. Days like this keep pushing his book into a distant future. It's time for the born-anew naturalist to pack up his gear and head home. For this winter's day, at least, there'll be no gift of light from the Chesapeake.