Terain-Water creation problems

Hello guys.. Im tryin to create a realistic and nice terrain.. i want all my terain to be covered of water(i have pro).. I put my water at the terrain.. after i create my mountains.. and when i add texture to my terrain then all the terrain covered with texture and the water doesnt look nice.. cause under the water you can see the texture of rock/hill (example).. How i can add texture only in some places of my terrain and not to all of it?