Many technologies are used to make watches water resistant, but the truly revolutionary step was adding an inner bezel that times the dive, and those timekeeping instruments became irreplaceable underwater.
Everything about those watches was designed to create the highest legibility, with large handset and indexes, and bright colors. The orange tint proved very popular, especially against a black background. Those watches were highly visible when worn at depths over a wetsuit; their large second crown controls the fundamental inner bezel, set to indicate the maximum time for a dive… More here.
The movement is basic and Dan Henry has a mixed reputation, but boy do these look good.