I like it. Cost less than quint but is only for water creatures - which makes sense.
It seems a tad bit hard to balance. If it only cost 1 upgraded, you could easily just make submerged dragon deck that would be pretty unkillable... I think someone mentioned that maybe dragons would be exempt from submerge as they are airborne?
Like the idea of having a spell for it.
Seems to call into question what the elements battlefield looks like (besides black hard wood floors)....