Based on where the player is standing (level of light surrounding them) it will take a effect on their inventory avatar making it very dark or effected by being underwater.
(Not sure if this is a duplicate of something since i do not know how to describe it)
1. Go to a highly lit up area and open inventory.
2. Note the lightness of the player in the Gui
3. Go to a non lit area (or very small lighting) and open inventory
4. Note again the lightness of the player in the Gui
5. Compare (Do this again in multiple lighting to see a big difference)
Not sure if this is a feature or a bug either ;/