The stuff grows like crazy and chokes off waterways and inlets.. pervasive in Florida, particularly in the inland canals and drainage ditches. Florida spends millions of dollars a year to control the plant, and as Rich points out, it's not just a problem in Florida.
As an interesting side story.. I remember visiting EPCOT many years back and they related their use of the plant and it's ecological benefits. Due to the amount of waste material produced from the theme parks and hotels, they had to find a way to solve their waste water problem. As part of their waste treatement, they have miles and miles of canals filled with Water Hyacinth. They run the waste water through those canals, and the plants, through their filtering root system and feeding on the waste particulates, pre-treats the water to the point where it is almost drinkable quality by the time it exits the canals and requires very little additional treatment. In addition, since the stuff grows so fast, they harvest tons and tons of the stuff and use it to produce energy to power the waste treatment plants and a good portion of the Disney complex. I'm sure I'm forgetting most of the details, but I thought it was an absolutely fantastic system they had in place and was very impressed at their ingenuity and use of renewable resources to solve a very large problem they were facing.
Cheers,
Brad