My Irish TB does it and he has taught one of the ponies to do it also, I don't mind too much, they're eating soaked hay so getting water from that and they don't seem to mind drinking hay flavoured water either
One of mine does it as well grabs haylage, drunk in water then happily munch, can make a complete mess of the stable, but i used to put water and haylage either side of the door so water ran out.
I dont have to worry now as he is happily on loan and making a mess of someone else stables and automatic drinkers
My ID does it and will happily dunk with automatic water drinkers so need to give him a dunking bucket. He will even drag his hay when put out on the field back to a water bucket if within a reasonable distance. Once the distance gets over about 7m, he tends not to bother. I have seen him go from haynet tied up outside one stable, go several strides away dunk, reverse back to haynet then repeat. Teeth regularly checked and turnout out 50% winter an 100 % summer and he still does it.
He has taught my other horse the joys of it although he is not as avid a dunker. It does make a mess but at least I didn't have to worry about their water intake/impaction colic in the cold snap. But we did have very pretty thick iced water sculptures inside the box from where he had dripped.