Ah, yeah, I can see how that’s not readily apparent right away.

There is tension & decision-space among the players. After all, if no-one takes any treasures, you could go all the way to the bottom and grab what you wanted. But as soon as people start looting, they start gasping and using up the tragically-common air supply. Some might deliberately try to drown each other by tactically picking up and dropping stuff. So it’s a reverse chicken race, it’s a game theory tense wonderful nightmare. All wrapped up in a super-classical roll-and-move shell that’s easy to play and teach.

(I was gonna recommend Deep Sea Adventures in this thread! But I see several people are already recommending it, and that’s awesome!)