Prismarine is a type of stone found under the water or in the monuments of oceans. You can stake the Prismarine from the monuments of oceans and then make the Prismarine wall.
The is a detailed guide that will brief you how to make Prismarine. Take a boat and go to the ocean monuments. Equip your iron pickaxe and break the blocks of prismarine to get six pieces of it.
Take your boat back and go to the crafting table. Open the 3*3 crafting grid and use six pieces of prismarine on it. This will make a prismarine wall for you.