I've never had an adult chicken drown or even get into a pool I have set up for my water fowl and I have 3 pools going, When I have tiny chicks I do worry and keep a large rock inside the pool in case one was to fall inside they could get out, I had a chick drown in a 2 gal water bucket I had set out for my water fowl though.
Ducks and chickens can basically eat the same things, but again housing them maybe a problem unless you make the coop large so the ducks don't have to sleep under roosts and get pooped on. keeping water and feed outside will help keep your coop cleaner but ducks poop alot so keeping the coop clean will go along away in keeping everyone happy, I use deep shaving and just scoop out poop daily fluff and it's dry by roosting time.
Then if you hatch or buy ducklings straight run you also run the risk of having drakes and drakes when hormones come around will try and mate chickens too, which is a death sentence for hens since ducks and chickens mate entirely different. So lots to think about before next spring comes around.
Any questions you have though we will try to answer for you, not trying to discourage you from getting ducks but doing the research first as your doing can sure help you and others make better decisions on if ducks are right for them.
Look forward to hearing from you.