NH Department of Agriculture and the herbicide vendor recommend diquat or fluridone as effective for treatment of spiny naiad. The only other lake in NH that has spiny naiad (Milton 3 Ponds) has used diquat for the past 2 years – they report good control, but not elimination. See the Pros/Cons document on the BIPC website for comparison.
http://bigislandpond.org/task-force-spiney-naiad-committee/