SOLVED. YA Coming-Of-Age Realistic. Main character (M) lives in a harbor city. Has a crush on his former babysitter (F) who he realizes is a burnout and possible drug addict. Possible Spoilers. [s]

[Anthony M.]

***Possible Spoilers Ahead***
I remember the ending of the book involved the main character being on a boat possibly with the babysitter and he realizes that the bioluminescent algae around the beach begins to glow.

Cover included water with a dark blue square containing the title with a small starfish over the title.

I bought the CD audiobook at a bookstore going out of business in 2006-2007.

*EDIT: I remembered that it involved the main character seeing a giant squid.

[Mai]

This one that came up in a search looks like a good fit!
The Highest Tide by Jim Lynch

[Anthony M.]

Thats it!!! Thank you Mai! I can't wait to re-read it now to see if any of my details are correct.

[Mai]

Glad this was it! Happy re-reading!