A couple and their friend are scuba diving off the coast of Puerto Rico. When they return, they find it extremely hard to breathe. They put their oxygen masks back on and return to their boat to find the captain dead. The three survivors head back to shore and find everyone in town - and maybe the entire world...are all dead!
Tropes are devices and conventions that a writer can reasonably rely on as being present in the audience members' minds and expectations. On the whole, tropes are not clichés. The word clichéd means "stereotyped and trite." In other words, dull and uninteresting. We are not looking for dull and uninteresting entries. We are here to recognize tropes and play with them, not to make fun of them.