Shoals are shallow areas of water, usually found near the shoreline, that are composed of sand, gravel, or rocks. Shallows are a type of shoal that is particularly shallow and often found in coastal areas. Reefs are underwater structures made of rock, coral, or sand that can be found in both shallow and deep waters. SANDBARS are also shallow areas of water, but they are composed of sand and are usually found in rivers or estuaries.