I just give them a foot bath just for splashing...
I have two mares that are trough divers. I have a big (100 gallon?) rubbermaid tank that we put under the fence so just enough sticks out into the pasture for them to get their head in and drink. The only drawback is I have to keep it topped off so they don't have to stick their heads way down in it. Makes it harder to dump and clean.
Then I have one of those short (about a foot high or so?) rubbermaid troughs in the pasture specifically for splashing. During the warm season I refill it morning and night and it usually does the trick. I also always fill it in the winter when we have days over 50 degrees -- especially if it is going to get in the 60's or higher -- with their winter coats on they get hot and I know will want to splash.
The mare looks like a strawberry half dipped in white chocolate, as if someone picked her up by her front half and dipped her in. I guess she was too dainty to get her head dirty. I always love the big wallow that gets mud on the center of the forehead-that takes some dedication and work.