You may well find it is easier to float java fern leaves until they produce some babies, which usually grow near the tips. They will have some roots. Then you can tie those roots to a rock.
The single fern leaves, if they don't have an attached rhizome, won't grow a new one. But many of them will live long enough to produce a few baby plantlets you can grow on, before they rot and die.
I refuse to buy ferns that are potted, since virtually always the rhizome will be rotten and I'll have a bunch of leaves that I have to wait around for while they make some babies, and then grow the babies on rocks or wood. It happens but it is slow, they are not the fastest growers even with CO2. And not all leaves make babies.. though most of them will.