Tubs have their advantages, as you mentioned.
My concern would be that tubs also suck heat out of water, out of little bodies, and ducklings do have very little bodies - a high surface to volume ratio, it is called - that means they lose heat easily. Body heat loss can be fatal.
My runners (eleven of them) were brooded for the first two weeks in a very large rubbermaid but (about 2'x4'), then transferred into a puppy pen lined with plastic poultry fence to keep them from getting their little heads stuck. I lined the bottom with 6 mill plastic.
I used towels for bedding and that is labor intensive but the rinse water from cleanup was the best fertilizer ever, and I am allergic to shavings. You can work out what bedding to use based on your needs. By the way, shavings are very dusty, too.
Another drawback to a tub (I am thinking you would have no bedding, because that would not rinse down the drain) is that slick surfaces can cause splay leg, an injury to ducklings' legs that is serious though it can be treated, if caught early. So here again, you'd need something to prevent splay leg, which would cancel the benefit of just being able to hose down the tub.
With two ducklings, I recommend you find Wifezilla's pictures and description of the brooder she set up - I love it, and might have done something like that if I did not have initially fifteen little fluffies (four belonged to a friend), and after two weeks, eleven.
