A sandbank is a raised area of sand in a river or in the sea, built up by moving water. It may hide just under the surface or stick out like a low island. Boats can get stuck on it. English also says sandbar for the same kind of ridge; sailors may say shoal when they mean a shallow patch that can catch a boat. Bank here is the 'raised ground' sense, as in riverbank — not a place for money. Though actually, speakers are a bit loose: a huge dry stretch the tide leaves behind, a thin ridge, and a shifting patch in a river mouth may all get called a sandbank. It is an ordinary geography and sailing word, not a fancy one.