The F. Piguet 1150, also used in many Lemans and, as you imply, a bit slimmer and more delicate than you'd generally want for a divers watch. A movement that slim, especially with double barrels, is going to have pretty narrow tolerances. The larger thicker movement in the 45mm Fifty Fathoms and the Bathyscaphes was made with diver duty in mind, and this one wasn't.