Why do things get darker when they get wet?
Two reasons:
1) Some of the light that bounces off of the surface of the object gets reflected back down by the underside of the water surface. So, it bounces of the object surface potentially multiple times before it escapes the water. That makes it look darker and more saturated.
2) the water surface reflects more like a mirror. A lot of the light goes in the same direction when bouncing off of water instead of going in all directions when bouncing off of a rough surface. So, if you stand where you can see the reflection of the light source, it will look a lot brighter than when the object was dry. More light in that specific direction means less light in all other directions. So, it's dimmer in most directions because it is brighter in one specific direction.
If you want a ELI-college professor, here's a pro game dev writing a series of articles about how to simulate wet surfaces in console games.
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