I haven't tried it with emersed plants, but I know that it only works for certain species of plants. There are several different kinds of rooting hormones sold for different kinds of plants. I am not sure if they work on aquatic species though...
Are you having trouble rooting a particular plant?
My understanding is that they are all basically indole-3-butyric acid or indole-3-acetic acid and that the only difference between them is a matter of concentration of this ingredient. They are indispensable for rooting woody cuttings (ie trees and shrubs) that root slowly. I have stuck everything from pointsettias to mums to all sorts of woody cuttings in the commercial greenhouse business. It was always the same stuff rootone-f and the f was for fungicide. I have heard of some hydroponic enthusiasts who add cut willow branches to their hydroponic reservoirs that serve cuttings for a few days in the belief that it will increase rooting speed.Technically there is some merit to this idea but I can not vouch for its efficacy in hydroponic systems.
I've tried it on several plants... I've had more luck with using it to propagate from cuttings that are already emersed than with converting submersed specimens. But, of course, different species respond differently. I've found Bacopa monnieri does well with it, as do a few of my Ludwigias. Hygrophila spp. and Staurogyne spp. don't respond at all...
If you want a rooting hormone for immersed I would check out SuperThrive, it's not super expensive but it's super potent, I've had a LOT of luck with it in both soil and hygro setups and there was a NOTICEABLE difference in both setups when using it compared to not using it. It should work fine and it's something that can be used all the time for the plants.
I have no idea on it's effects on fish/invertebrates tho so haven't even tried it and don't plan to on an actual aquarium.
willow cuttings are packed full of I3B. I've used willow cuttings for curing green water but at the same time my egeria densa and a sword I had floating threw out tendrils and roots like crazy. It could have been totally unrelated but then again maybe not.