I've never used a bucket heater or a store brought insulated bucket before...In the paddocks I have the tub like muck buckets for water, and I also, have 5 gallon buckets in the stalls and sheds so that there are 2 drinking options for the horses. And I make sure that they have access to both buckets at all times through the winter. During the spring though fall, I take out the stall buckets, and only put them back in when they are going to be shut up in them.
In the shed and stalls I keep the 5 gallon buckets in a corner of the stall, sitting on the ground with the handle mounted to the wall with a bucket mount, so they don't tip. When it is going to get below 45 I push shavings around the buckets to help insulate them. I also have heat lamps in the sheds and stalls, it helps keep the water warm along with the horses.
For the outside buckets, I only keep them filled about 1/2 way through the winter. In the morning I remove the ice, and refill with water. We have a well, and the water from it is always pretty warm during the winter so it helps to warm up the buckets.
I haven't tried floating tennis balls, with the clowns I have I would never see the tennis balls again. Well, atleast not until I mowed, lol.