Having a drink

I will never understand horses. My mums horse featured here, is kept in a section of a field, to stop him getting too fat on grass. Only thing is it worked to well and he was getting a little too skinny, so we have put him in to another bit of the feild (more grass) to bulk up ect. He has a big bucket of freash water in his field (he knows its there) but yet he goes straight too the slightly dirty waterbutt when returning him to the original field, as if he was dying of thirst, stupid horse!