Last Woman on Earth (1960).
Second movie I’ve seen with a woman named Ev in it, and two for two, she’s an unhappy alcoholic trapped in a loveless marriage.
Note to self, never change my name to Ev, and apologies to anyone reading this whose name is Ev. Sorry!
Ev is diving with her rich old husband Harold and a young handsome lawyer Martin when the world suddenly ceases to have enough oxygen to breathe and everyone dies. Except them, obviously.
And then we get to see the steady disintegration of what could be a very idyllic existence on a tropical island into silly petty jealousy over who gets to tup the only woman left alive.
Not even being a Rifftrax could help this sorry excuse of a story.