The ocean water in Cocos island is always around 27 °C
(80.5 degrees fahrenheit), which is a pretty warm temperature to go diving in.
From December to May, (the dry season,) the water visibility is about 30 meters, also a decent visibility.
But sometimes there’s a lot of plankton, and the visibility range may drop down to about 5 meters because the plankton blur the visibility. But that would be a great time to see a whale shark, because they eat plankton so they’re attracted to it so you might be able to see better when they eat all of the plankton surrounding you.
All in all, the visibility is normally around 10 or 30 meters or somewhere around the middle.
The depth is 33-130 feet deep (10-40 meters).
And if you go closer to the surface, about 0-15 feet below the surface, you might be able to see some kind of ray like eagle rays, which are very popular around Cocos, jumping above the water in large groups of about 10.