To stretch the simile a bit and take it in a direction probably unintended by the original author, fish don’t just live in an environment full of water; water is inside them at all times.
In other words, everyone is religious—if I can take sense 2 of religion from the New Oxford American Dictionary:
“A particular system of faith and worship: the world’s great religions.”
Everyone has a particular system of faith and worship, atheists included. You either believe and worship the one true God or a collection of idols you have chosen to replace Him (Rom 1:18ff). Everyone believes things he can’t prove except with circularity. Everyone makes something or someone his ultimate basis for belief.