Can sperm in precum swim in pool water and go up the vagina impregnating a girl?

Nope. Hi. Nope, you're safe! When we talk about "swimmers" we mean the short distance in the vagina to & thru the cervical os, the uterus, and into a tube...not swimmers swimming in a swimming pool!
Doubtful. That would be some pretty determined sperm. If the male ejaculated right by your vagina without any barrier it may be possible but highly unlikely. As always if you are concerned and miss a period. Take a pregnancy test.