Watercress you can buy at the grocery store,
if your store doesn't have it you can have them
order you a box (last time I did it I got ten bunches
for $10) Place a stem under a rock. Doesn't matter
if is comes with roots or not. It will wilt a bit and
then bounce back.

Water hyacinths are illegal in the southern states
as they clog natural waterways.
In the north they are usually for sale at pond
nurserys because it won't winter over outside
in the north.

k :-)