Well, personally i would get two ducks not one, while the chickens will provide company, ducks do have their own quirks and only another duck can participate.
Ducks are water messy, they adore mud that said that are quite hardy, disease resistant, excellent foragers and so forth. The ducks(females) are definitely louder with the only exception being Muscovy, they mostly trill, coo, chirp etc.
Flying varies on the breed, i have two that do, both my calls and my Muscovy, the buffs not sure yet they are just wee ones. i hear varied things on their ability.
I leave my ducks full flight but my Muscovy, free range so i think it's a line of defense and won't remove it that said my drakes once they hit 1yo? nah, they don't anymore, too much bulk.
In a tighter living area, you'll likely want to clip a flighty breed, do you have Storey's guide to ducks? he shows how in it, same as done with say, a budgie.
I would recommend this link on the care of ducklings so you know what your getting into...
https://www.backyardchickens.com/t/750869/raising-and-caring-for-ducklings#post_10611711
Biggest disadvantage of ducks vs chickens? mess, ducks win the prize that said i think they are worth the trouble, i am more ducks here than chickens but it depends some find the work load and differences frustrating and not something they care to do.
and yes, kiddy pools are just fine and actually preferred by me, i can keep them quite clean and have no risk of water predators.
