On the inner surface of the ciliary body is a series of projections that secrete the intra-ocular fluid, thus keeping the fluid pressure up inside the eye. The fluid is diffused into the blood or drained through a canal at the same rate it is produced, thus maintaining a constant ocular pressure. Fish do not have ciliary bodies, in part because fluid is all around them. To maintain water balance inside and outside the eye, freshwater fish absorb fluid into their eyes by osmosis directly through the cornea. Any excess salts from the eye are removed and excreted in their urine.
Sharks maintain a high level of urea in their blood and eyes.
This allows them to keep osmotic pressure inside their eyes higher than that of the surrounding salt water, thus keeping the eyes firm. Marine bony fishes have a lower salt concentration inside their eyes than the surrounding ocean. This means that the eye should theoretically shrink or collapse due to loss of water from inside the eye by osmosis. Somehow, fish manage to maintain fluid pressure inside their eyes despite this osmotic difference, but the mechanism is still a mystery. Water not only reduces the amount of light that penetrates a fish's world but also the kind of light.