The plants which grow submersed are almost necessarily either submersed only in nature, or are bog plants that can grow emersed, but only in very wet conditions. I think that is true. If it is, your HC experiment might have worked well if you had used enough water on the soil to totally saturate it, enough that any more would just be standing water on the surfacee. Then if you had covered it with some plastic wrap or equivalent to keep the humidity at near 100%, and to retard evaporation, it would almost certainly have worked. These are not plants to be grown as ordinary garden plants, where you need to avoid having the soil too wet.