Can't tell, I'm French.
Info from The Guide :
HOW DO I CATCH JESSICA SWIMMING NAKED IN THE POOL AT NIGHT AND HAVE SEX WITH HER?
Jessica likes to swim naked at night in the pool between midnight and 2:30am. Randomly might catch her doing this providing she isn’t the night receptionist. Don't leave. Once Jessica notices you, she will swim over and ask about her breasts. Don't say anything, just continue to stare. If her affection is too low or you don't get an erection, she will get mad at you and leave. If her affection, love and lust are high, and you get an erection, she will see that you just can't get enough of her boobs and ask for her towel. Give it to her, and she will have sex with you on the sunbed. If you refuse to give her the towel, she will tease you and then leave. Nothing left to do, but masturbate on the sunbed, thinking about Jessica. Yet again what a tease.
Side note: If Jessica's affection and lust are high, and you tell her that her breasts are just the right size, she will accidentally let her towel slip and then leave. While this nip slip is nice to see, it isn't enough to really qualify for an achievement.