If you've ruled out shedding and mites there are a couple of other reasons why he could be in there. He feels more secure in the water dish than his hide (do you have an appropriate sized hide for him) or he is dehydrated (check your humidity levels). Snakes will soak if they have injuries or skin irritations, etc give him a good looking over to rule out that being a cause also. And make sure that you keep his water clean.
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Tosha 
"Stupid is forever, ignorance can be fixed." Don Wood
"Of course, that's only my opinion...and I believe I am God." =) Chris Bianco
7.33.0 Ball Python (Harry and Fluffy and gang)
1.0.0 Angolan Python (Skywalker)
0.0.1 Green Tree Python
0.2.0 Feline (Pippen and Pandora)
0.0.1 Dessert Tortoise (Pope John Paul)
2.2.1 Fish (1,2,3,4)
0.0.0 Pool skimmer rescues for this season