Oblong shaped board with two cut-out handles at each end and three items attached to it: a compass, a watch and a depth gauge.

Covert operations in the 1950s and 60s.
How did UK military divers swim undetected some distance to a location to investigate a target underwater?  

Well, they used an oxygen rebreather (so as to leave no tell-tale bubbles) and navigated using a swim board. 
A swim board was a wooden board to which was attached a watch, a depth gauge and a compass. The diver would use the compass and watch to work out the direction and how far he had swum; and the depth gauge to ensure that he didn’t go too deep and get oxygen poisoning.

The swim board was usually hung around the diver’s neck with a length of cord. Handles were cut into the board at each end to make it easier for the diver to hold and view the board underwater. The swim board was used to enable the diver to accurately approach the target.

In the museum look up and see our example.  (Don’t be tempted by the “Rolex” diving watch; it is a fake!).