SOLVED. YA Novel about a girl named Jessica who goes on vacation with her dad and his girlfriend [s]

[Jessica]

I'm pretty sure this book had the name Jessica, its part of why I read it when I was a kid.

Alright this one is about a teen girl who goes on a vacation with her dad and his girlfriend. I remember she was resentful towards her father, and when she first meets the girlfriend, she's apparently only her dad's friend. I remember a section where they talk about the woman wearing "a no nonsense black bikini" and the teen girl mildly admires her for it.

She also sees her father and the woman in a Jacuzzi or pool either making out and having sex.

I think they went on vacation to mexico, I recall something about her getting sick from drinking the water.

[Tab]

Unfinished Portrait Of Jessica ?

If not, how many years ago did you read this?
Anything about the cover?

[Jessica]

That's the one! Thank you so much!