I don't have any help for your problem of supporting aquatic plants, though it would look great if it could be somehow encapsulated in 3-D. However, in addition to supporting aquatic plants outside of their natural environment, you also have to find a way to preserve their natural wet appearance, and also their color. Maybe you have already worked that out, but if not, look at Curator, Vol 35 No 2, "Preservation and Coloring of Seaweeds." That process was developed for an exhibit of intertidal zones on a rocky shore. After 3 years, the plants still look completely natural. Let us know if you come up with a solution for support.