There's always the New England Aquatic Plant Society - http://www.ne-aquaticplants.com/
We just had a meeting at Ned's store last month (lots of free plants - not sure if Ned liked that too much) and we're trying to move the meetings around since we have members all over NE. Unfortunately the next one is Springfield.
We also have a few plant people in the Tropical Fish Society of RI - http://www.tfsri.org
. Mostly fish people, but I'm trying to turn that around.