Is it possible to drown a fish and how?

Yes, it is possible to drown a fish.

What you need:

An airtight box

A fish

Yourself

Go to a lake, ocean, or place with very deep water. Before you jump in, place the fish in an airtight container with NO WATER. Jump in the water with the airtight container and drown. You will drown with the fish, but yea, who cares.