What do you call someone who doesn't know how to swim?

The word or someone who cannot or does not swim is non-swimmer.

I am not aware of a word for a non-swimmer who dives into try to save a drowning person. I would call them a hindrance or "a danger to themselves and others", as they will only make things worse and will mean that rescuers need to save two people, not one.

According to this thread, the word "landlubber" can be used (mainly by older people) as slang for someone who can't swim, and "the Rock Squad" is US military slang for such people.

You should be careful when using Matt E Ellen's suggestion of "non-swimmer", since this word can also be used in a racist sense - see Urban Dictionary.