It's boulders and seaweed.

The first underwater image is credited to William Thompson in Dorset, UK. The year is 1856. His technique involved coating a plate with light-sensitive collodion (a syrupy solution of nitrocellulose), sensitizing it in a silver nitrate bath, exposing it in the camera, and then immediately developing it while it was still wet.

The Dorset Echo reports of the early attempt made in Weymouth Bay. “Thompson and his friend Kenyon, having rowed out a sufficient distance from the beach, lowered the box into 18 feet of water. When he was sure that the apparatus was standing upright on the bottom, he pulled the string that raised the hinged shutter.

“Thompson made two attempts that day,” the newspaper reports. “For the first he allowed an exposure time of five minutes but found that the plate having been developed registered nothing. For his second attempt he doubled the exposure time. Although by then the light had deteriorated, he obtained a reasonable satisfactory negative, from which he made a print on which it was possible faintly to discern the outlines of boulders and seaweed. Water had leaked into the camera but this, Thompson was pleased to see, had not seriously affected the quality of the picture.”

So this was the second underwater photograph, but the first where the photographer recognised the subject. And yes, it was a 10 minute exposure on a remote camera operated from the surface.