When her shark-diving cage breaks off and sinks to the seabed, a first-time diver must resurface amidst a looming great white before her oxygen tank runs out. [In The Deep, 2016]
The IMDB entry describes the film as two sisters trapped in the cage . So here is my take with co-protagonists:
When their shark observation cage breaks off and sinks to the bottom, two sisters, first time divers, must resurface amidst looming great whites before they run out of oxygen.