Hi Pope,
Your pond may be too volatile temp wise, for water hyacinth to get going large.

Full sun and a small volume pond will probably scorch and keep them growing at a smallish size, they do struggle in temps whistling over 95°f and hot midday sun until Summer cools off. A bit of shade will help them through the heat of midday.

A night bloom tropical waterlily is a pleasant alternative for providing shade in a very hot sunny position, fairly easy to find on ebay for $10 or so

No, theres no restriction on water hyacinth in NC, I list spares of both on ebay now and then

http://search.ebay.com/_W0QQfgtpZ1QQ...QsassZadavisus

Regards, andy
http://www.flickr.com/photos/21940871@N06/
http://i93.photobucket.com/albums/l42/adavisus/