Before analyzing the ending of Black Sea (2015), it’s worth noting that there are a few details of the film’s ending (and beginning, and middle) that simply don’t work. Suspending disbelief on those subjects (as in, who pulled the inflation cord on the suit full of gold?), there are seemingly legitimate reasons why Robinson (Jude Law) had to go down with the ship in order to save the others.

One thing Black Sea doesn’t do well is explain the way anything in a submarine works. Watching the crewmen talk about ship-stuff makes you feel like Tobin (Bobby Schofield) did when he had all that 1-2-3-left-right valve wheel turning business explained to him. As such, it’s often confusing to follow the actions of the film’s characters. That’s not to say injecting expository explanations of boat procedures would have been the right thing to do - it wouldn’t make sense in a sub full of lifelong seamen - but something to let the audience in on their business would go a long way.

Nonetheless, the takeaway from the final scene is that the three remaining men were trapped within the boat’s torpedo room. The tubes the men escaped through were, naturally, airlocks designed so the submarine wouldn’t get accidentally flooded by their use. It seems the outer hatch on the tube could only be opened from within the submarine, after the inner hatch was shut. That way, no water gets inside.

After the two men float to the surface, Tobin says something about Robinson coming up behind them by using the emergency lever. This, ideally, would be a lever within the tube that would allow the outer hatch to be opened from within the tube, letting someone get out if they were trapped inside. No such lever actually existed, however, which is why Robinson was stuck in the sub. The fact the outer hatch had to be opened from within the vessel is also why Robinson couldn’t have merely suited up and waited for the room to flood, opened the hatch, and left. There’s no way to get both ends of the tube open at the same time.

For a less literal and more analytical interpretation of the question, “Why couldn’t Robinson escape,” one can examine Robinson’s sense of responsibility. He led these men on a suicide mission that was, unbeknownst to him, a setup from the beginning. Almost all of the crew had perished. This was his fault, and in the honorable tradition of a sea captain, he felt the need to go down with his ship. In a last act of redemption, he sent some gold bars up to the survivors in an empty suit, and awaited his fate.