This is an air pump affectionately nicknamed "R2-D2" by the SCUBA diving community. It sits in the engine room of the Fujikawa Maru: a cargo ship sunk in 1944 as part of Operation Hailstone during WWII. Operation Hailstone was the Japanese equivalent of Pearl Harbor. Allied forces sunk numerous ships inside a maintenance and supply depot in Truk Lagoon, (now named Chuuk), in the Federated States of Micronesia. The ships in Truk Lagoon were rediscovered by Jacques Cousteau in 1969 and documented in 1971 as part of his television series "Undersea World" in the episode "Lagoon of Lost Ships".
To get this picture I had to work around a metal barrier that sits exactly where I needed to be, and there is not enough room to go around it. This barrier has holes in it large enough for your hands but not your camera. So, I gave my camera to the dive guide who handed it back to me after I put my hands through the barrier. Once I got the shot, we reversed the process.
As you can see, there is a lot of debris in the water: mostly kicked up by divers. I would love to go back and try to get a better shot, but that is not likely to happen. In 2010 when I took this photograph, the engine room was still easily accessible; however the super structure has since collapsed, and trips into the engine room have become very difficult and not recommended.