The weed was identified as Giant Salvinia, a water fern. To check the species of the Salvinia auriculata group, look at the key by Barbara PARRIS (Fern Research Center, New Zealand). The sporocarps are very easy to check. It is impotant to do this before considering biocontrol, which is very efficient against this weed. But it will be surprising if the species is not Salvinia molesta DS Mitchell (Giant Salvinia).
The shape of the “hairs” on the surface of the plant are all very distinct.