I've seen some ingenious designs for solar heated waterers that don't freeze, you might search around on the internet to see what you all can find.
Here, we do have power to our duck runs. I use metal buckets with bucket heaters and pare down to two pens for night time use. The bucket heaters will keep the water thawed right down to below zero, though there will be some ice and I add warm water at times. We have a freeze proof spigot near the duck buckets, but we're having issues with it freezing, darn it! We dug it out and found the drain field was compromised with a lack of gravel and tree roots.... thought we'd fixed it, but it froze a few days ago and it only works when it warms up in the afternoons... I think its worse than it was before we "fixed" it! Looking for a repair kit, since it seems to be more than the drain field. At least it quit bubbling water up next to the pipe.
For bathing, I wouldn't even try to keep a kiddie pool going here. It gets below zero (down to 20-30 below sometimes) and it would just ruin the pool. We have concrete contractor's tubs and I put one out under the sort of freeze proof spigot and let them bathe on the warmer days. The ice easily dumps out of those tubs, they are inexpensive (I have several of two different sizes) and they are tough. The ducks don't have any problem with bathing in cold weather, their feet could possibly freeze, but don't seem to. They are designed to migrate at high altitudes in cold weather, they pull their feet into their down and are fine. In fact, they don't seem much bothered by bad weather.... though I do lock them in the house if its windy and cold, otherwise they have access to a house and a yard (no heat) with lots of deep straw, which they love. I've never lost one due to cold weather, though one poor boy who had a leg injury that I thought had healed, slipped on the ice and reinjured himself so badly I had to put him down. They make ice with their feet - I keep straw on hand to keep putting over it all winter when its too frozen to clean. In the spring I have a big mess to clean, but they are comfortable all winter.
For 25 ducks, part of the year we have to carry the water to fill their buckets. It's a lot of work and I'm glad my husband helps, because I can't carry a full bucket. We go through 6 to 9 gallons of water a day for drinking and more if they bathe. It's one reason I shoot for no more than 20 ducks before bad weather, that and more than that would need more than two pens, really adding to the mess and the work. I don't think we'd be able to care for much more and do a good job, so I limit myself.