IPA: /ˈswɪmə/
KK: /ˈswɪmɚ/
A person who swims, especially as a sport or for enjoyment.
She is a talented swimmer who competes in national championships.
Swimmer → It is formed from "swim" (from Old English *swimman*, meaning to move through water) and "-er" (meaning a person who). A swimmer is a person who moves through water.
Think of someone who 'moves through water' — that's what a swimmer does, and the '-er' just indicates a person.