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.