I've heard that many mosses have both aquatic and terrestrial forms. In fact, I believe java moss is one of them. So I'm hoping spanish moss is the same way. I have no idea if java moss is legal to ship here or not.
I've never ever seen Spanish Moss in the water. I'm 99% sure it will simply rot, but I would love to see you prove me wrong . Just make sure you test it in a tank or bucket or somewhere it won't ruin anything.
I would say Spanish moss is terrestrial. I lived in southeast Arkansas when I was younger. The moss was everywhere in the trees, but I never saw any live moss in water.