Correct - praziquantel is poorly soluble in seawater, and only a little more soluble in freshwater. Here is the thing though - even when it is dissolved in a solvent, if you look closely, you'll see that it comes out of solution right away when added to the tank! It turns out that the fine particles in the water work - so using a fine mesh net to mix the particles into the tank works just as well, without the issues that the solvents can cause (bacterial decomposition that suck oxygen out of the water).
I've not used Prazi-Med, but their ad says it is 100% pure praziquantel, so that would be fine to use.