Him being in the water dish for that long of a time may be caused by
a) Mites. Wich if that is the case, has to be treated immediatley
b) You have the enclosure too hot and he is trying to find a place to cool down
Thats all I could think of at the moment. You should check for mites. And if you havent already, measure the tempurture of the cage with a digital thermometer (the stick on thermometers suck) And regulate the Under Tank Heater (UTH) with a thermostat or a rehostat.