Is there one word for someone who does not know how to swim?
Even better if there is one word for someone who doesn't know how to swim but dives to save a drowning person?
If no, then suggest a similar phrase.
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.