The Eye is the Lamp of the Body
"The lamp of the body is the eye. If therefore your eye is sound, your whole body will be full of light.
But if your eye is evil, your whole body will be full of darkness. If therefore the light that is in you is darkness, how great is the darkness!"
This doesn't mean that Christians can never go to the beach or to the pool. But it does mean that while we are there, to be radically different, and not look at things with our eyes in the same way the world teaches us to look at them. That is — with lust, envy, desire, longing, and jealousy.