There's also this: https://en.wikipedia.org/wiki/Monocular_deprivation "They found that ocular dominance columns (the orderly clustering of V1 neurons representing visual input from one or both eyes) were dramatically disrupted when one eye was sewn shut for 2 months. In the normal feline, about 85% of cells are responsive to input to both eyes; in the monocularly-deprived animals, no cells receive input from both eyes. This physiological change was paralleled by dramatic anatomical changes. The layers representing the deprived eye in the lateral geniculate nucleus of the thalamus are atrophied." So covering one of your eyes for long periods of time will weaken it. Something tells me the Hylotl would be SUPER against the weakening of their third eye. At the end of the day, when you're making racial armor you should put yourself in their position. Try and imagine what it would be like to have fins and three eyes and prefer to be near or in water.