So I have a question about Styrofoam

so I want to make my land and section out of Styrofoam and silicone peat moss and bark on it and it will be submerged in water so will it realsse chemicals and kill any fish or just slowly decay if yes how do I make it water proof and I also want to make a small water fall and it need support so I want to use styroafoam and silicone rocks and slate so I just want it to be safe