For watercolor there are several paper and or boards as I listed in the paper page in this menu.
For oil painting I like to paint on Masonite board covered with gesso medium, sanded lightly. If you decide to use pre-made canvas board I suggest painting over it first with gesso medium, sand lightly or make what is called rabbit skin glue.
Rabbit skin glue is exactly what the name states. This takes a bit of time to prepare; Rabbit skin glue comes in a powder form ready to be cooked. Follow directions on the bag, but all you need to do is boil the powder with water until a paste like consistency. Once the glue feels like pancake batter consistency it is ready to be used. Simply paint the masonite board and let dry before use. You can sand lightly. Rabbit skin glue is what The Masters used to prepare their boards.
Important: Do not use turpentine to thin down paint! Turpentine breaks down the pigments in the paint and will discolor over time. If you want to thin the paint down use linseed oil or an oil thinning medium.
As far as acrylic paint is concerned there are mediums made just for thinning, you can use water but the paint becomes transparent.