Minecraft resource gathering/Clay
Clay can be found in riverbeds, lake floors and in shallow oceans, and can be mined with a shovel (or your fist if you fancy drowning) to get clay balls, which can be smelted into bricks, or crafted back together into clay blocks, which can be smelted into terracotta.
It may help to use a potion of water breathing or enchant your helmet with respiration if you plan to mine clay in large quantities.
If you use Silk Touch, it drops the clay block.