According to American Pregnancy it looks really improbable. Similar statements are made on the Oakland University website and on this website about contraception (which explicitly states it's impossible). The girls should have been swimming naked and the "father" should have ejaculated just outside her vagina as sperm would be killed in a few second in the pool's water and it would not be able to "swim" in a pool. It is more likely she actually had sex and tried to hide it from her (really gullible) parents.