I'm ok with terrain in general... if for no other reason that it forces some tough decisions on spell book building. What I absolutely hate is that Shallow Sea has Cantrip. It's not like the Siren isn't going to have 3-4 of the things anyway....
A Dispel Terrain / Blank Terrain / Stone Floor or whatever to get rid of unfavorable terrain isn't a bad idea, but probably not needed in my opinion. I would cost it at "X" (cost of terrain being dispelled), but would not make it novice. Maybe make it Arcane 1 or Nature 1?