Testing is the way to go, sure. For example, Riccia flutans and Riccardia chamaedryfolia are found in nature on muddy substrates but few or no guides will tell that they can live as aquatics, I suppose it sound very strange when someone attached them for the first time to a stone and submerged them. And it worked, for these species there is a real possibility of underwater living, they can go beyond these "very wet conditions or water splashes". But no one knows, there are hundred of species that live in that water limit, although my guess is that we will see in the coming years more species being incorporated to the hobby.
Jordi