"A bright appearance in the horizon, under the sun or moon, arising
from the reflected light of those bodies from the small rippling
waves on the surface of the water."
--Bowditch´s Navigator, 24th edition (1854)
'Kumatage' appears to be a comparative neologism, coined, we must assume, in the early-to-mid 1800s. It appears to be formed using the Greek kumat, meaning wave, and -age, almost certainly stolen from mirage (it is worth noting that 'mirage' first appeared in English in 1812). It does not appear in any standard dictionary, although UrbanDictionry does list it.
This word is interesting, in large part, because it is very rarely used to refer to actual reflections on the water. Instead, you will much more likely encounter it when you stumble across the Kumatage Cafe, the Kumatage lounge, or Kumatage Lane. You may cross paths with Kumatage sunshades, a yacht or three named with variants of Kumatage, and any number of beach-adjacent businesses that would like to be associated with bright aquatic reflections. This would be understandable aside from the fact that no one ever uses, or even recognizes, this word.
Reflections of the sun on water may also be called a sunglade or sunwake; likewise, reflections of the moon are called a moonglade or moonwake.