Wow, you've been busy! I have never come across that Myriophyllum on Oahu. the only thing close are Ceratophyllum demersum and a Najas species found in the irrigation ditches that run thru Mililani.
You should seriously look for mosses and liverworts since you live in Hilo. I bet you'll find a bunch of stuff that grows submersed being that it is so wet over there. Just take a small peice and put it in shallow water. You'll know if it is submersible in about 2 weeks.