Sedges that grow without soil, their roots grow different. The roots on my yellow flag appear to be like long dense feathers. Roots that grow in soil are different, and it will take time for the sedge to "plant" itself in the water. Cattails should float too once the roots are properly established in water. If you take a cattail bulb and throw it in the water, it too will float, cause the bulbs are boyant, so whole plant may also float as well once the base is established, although I have not tried growing a cattails without soil.
When I used to make water plants years ago, I would chuck sedge bulbs in water that needed to be planted up, and noticed that most water plants the bulbs are boyant and float. Then a long time ago I read about sedges that would break off during a big storm and float to the other side of a lake, and appeared to be large floating islands. I originally had the yellow flag planted in river muck/clay that I got directly from under the bridge, down by the river, and had the yellow flag planted in a basket in that, 22 years ago. It grew very large the first two years and the basket broke away. The yellow flag was so big and I noticed there was no soil left, the plant must have ate it, and then I just stuck it back into shelf along the waters edge. The yellow flag grew so big it crossed the pond to the other side! I noticed that the fish would swim under the big plant and hide in it's long feathery roots, and showed my friend, who told me that the flags float! After a few years I made the pond bigger and then rolled the big clump back into the pond cause it was too heavy carry, and noticed that big clump that must have weighed over 250 pounds, floated once I set the root ball back into the water, and then floated it to the other side of the middle pond, where it now is too large for me to lift out of the pond, so I cut away pieces in the summer from the edge to keep it trimmed back.
Pictures from last year, could not find a proper shot of the feathery roots, but you can see to the bottom of the pond and the edge of the sedge that floats.