Having used hormones for several years back 5-6 years ago, they don't do anything for submersed plants.
There's some evidence they do help some Crypts flower in emergent culture but it's a one time affect.
Mike Kane at UF is one of the leading Tissue culture researchers for aquatic plants. He'll tell you the same thing. We will be stopping by the lab there in UF this June on the plant fest. You can ask him yourself.
FYI, Super thrive is about 1.89$ for a bottle at most any garden center. You can try it till you are blue in the face. But you'll not see the difference, terrestrials are somewhat different.