I've grown reginae outside in the ground, in regular garden soil and in pots in regular garden soil. I also grow them outside in slightly raised beds filled with ProMix HP and which are underlaid with regular garden soil. While they do like to be in very damp to wet soil, I doubt "submerged" plants will last long. Being submerged was probably something that happened after the plants got established in that location. I bet they eventually drowned if they stayed submerged for long. That being said, seasonally being under water for only a few weeks probably works out okay. However, they are not aquatic like water lilies or even marginals like cattails. Yes, they like very damp to wet soil; but, I doubt if the roots are literally under water for long that they'd survive from year to year.