Feeding fish at the blue lagoon.

This is a euphemism for going to the toilet - the blue lagoon is reference to the use of blue tablets that colour the water in the toilet.

Sorry I was late, I was feeding fish at the blue lagoon.

Fuck em and feed em to the fish

definition-
fuck them

quoted by Trenton T Curtin

Person 1: your friends are assholes.
Person 2: Fuck em and feed em to the fish.