BLUE LAGOON. BtVS Intertext/Allusion.
In "Go Fish" we find the following dialogue, in which Xander and the other Scoobies talk about his fear that he may be transformed into a monster like the other members of the Sunnydale High swim team:
GILES: They're absorbing the steroid mixture through the steam.
XANDER: Not they. We. Me! We have to find an antidote, don't you think? The clock is ticking, people!
BUFFY: I wouldn't break out the tartar sauce just yet. I mean, it's not like you were exposed more than once. Twice?
XANDER: Three times a Fish Guy.
BUFFY and WILLOW: Oh...
CORDELIA: Whoa.
XANDER: What am I gonna do?
CORDELIA: You, you, you. What about me? It's one thing to be dating the lame unpopular guy, but it's another to be dating the creature from the Blue Lagoon.
XANDER: Black Lagoon. The creature from the Blue Lagoon was Brooke Shields. And thank you so much for your support!
BUFFY: I think we'd better find the rest of the swim team and lock them up before they get in touch with their inner halibut.
Blue Lagoon was a 1980 film starring Brooke Shields. The IMDB offers the following brief summary:
Two young children are swept up on a desert island after a shipwreck. The film follows them as they grow up and become more mature, without the intervention of grown ups.
