Your ducklings are cute! I do believed you need to give them access to drinking water at all times though, as they need it to swallow their food. The cool whip container idea is a great idea, or a butter tub or anything like that. Especially if it is that hot there, they need water. Even though they make a mess, it is still necessary. As for swimming water, put them in a shallow pan or something like the upside down cake cover and stay with them and watch them. Then towel then off and put in warm brooder when done. I only have one right now. She is two weeks old tomorrow. I have been letting her swim in my kitchen sink!! LOL. It is stainless steel and easy to clean up when done. I just put the plug in, run lukewarm water (not too hot or cold) just enough so she can stand up good in it. Then I put her in it (AND NEVER WALK AWAY OR LEAVE HER ALONE). They are very fast and can get out and off the counter before you know it. Anyway, I play with her in the water and throw peas in there for her, etc. She loves going under the water for peas. It is so funny watching her. I only let her stay in their for about 10 minutes, then take her out, dry her off, put her back in the warm brooder.
In the brooder itself, she has a chick waterer that she can drink from. She does make a huge mess sometimes, but I use paper towels as bedding and it is very easy to change them out. My brooder is a large, plastic rubbermaid type tote. I have paper towels in the bottom, a beanie baby toy, a rubber ducky (for friends for her), a small food dish with her crumbles in it. On top is a screen secured with bunjie cords, then a towel over half of the tote, then the tote lid on top of that turned sideways to cover only 2/3rds of the brooder (to protect her from my cat). Then I put two heavy books on top of that so the cat cannot knock the lid off of it. Oh, I also have a heat lamp shining down into it (clamped on to the side of the tote) to keep her warm. It is on the end where the tote lid doesn't cover it. To get her out, I just move the books, take off the lid, move the towel and lift a corner of the screen. When done, I tuck the screen back into the bunjies. It sounds complicated, but it isn't really. I just have to make sure Xena, my cat, cannot get into the brooder with her. She is very interested in the duck. She even lays on the tote lid. I think she likes the heat lamp.
Good luck with your babies!
Kathy