I don't know if it can float freely. The Water Celery I bought was fairly heavy in a pot of tight clay so that it could sit on an underwater shelf and the plant was about 16 inches tall and 12 inches wide.
Water Celery I bought from the greenhouse, which I bought all they had, was just planted in some really tight clay soil in a pot. I just got a wider pot, that was a little shorter than the original pot, and drilled 1/8" holes all over it. I figure the wider pot gives the plant more room ... I guess, it might not. Anyways ... The tight clay soil it already was in stayed together very well, none of it floated out except for a very tiny bit when first putting it in the water.
I bet ya can just use the generic walmart clay kitty litter, red/blue bag, or I figure ya can use pea gravel. This stuff just keeps the plant anchored and I guess can help protect the plants roots from fishies get'n to them, but the roots will eventually start poking out of the holes in the pot, which is ok.
I am told the water should only go up to the top of the root ball.
In my situation, the water only wetted the bottom half of the root ball. So, I used a small 400 gph fountain pump to feed a manifold that feeds 8 or more 1/4" tubes where I had one tube per plant to wet the top half of the root ball. This is probably not necessary tho. I don't know. The manifold I am talking about is a drip irrigation part that ya screw on a 1/2" pipe thread, normally where a lawn/garden sprinkler is, but I put it on the fountain pump.
Quick heads up ... It took about 2~3 weeks for the water celery to wake up from its transplant shock. Once it transitioned, it got really green and started new growth from the root ball.
I never did anything to fertilize it since the greenhouse people said there was a fertilizer mixed into the tight clay soil.