Question

•Why don't our eyeballs fill up with water when we swim?

•Are there nuclear reactions going on in our bodies?​

Answer 1

It is to protect it from other chemicals present in water

Answer 2

This fluid provides pressure that helps the eyeball keep its shape, much like how filling up a balloon with enough water will help it keep its shape. Even if there were an uncovered hole leading into the eye, the eyeball would not fill up with water when we go swimming because it is already filled with this gel.