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.

Fish Feeding

A sex Position where your Partner is submerged underwater, while you cum. Then your Partner has to try eat it like fish food.

My and My girlfriend tried Fish Feeding, She nearly Drowned