Sea anchor
Modern commercial sea anchors are usually made of cloth, shaped like a parachute or cone, and rigged so that the wider end leads and the narrower end trails. When deployed, this type of sea anchor floats just under the surface, and the water moving past the sea anchor keeps it filled. Some varieties are cylindrical, with an adjustable opening in the rear that allows the amount of braking to be adjusted when deployed.
Sea anchor
Anything that can act as a source of drag in the water can act as a sea anchor; a common improvised sea anchor is a long line (a docking warp or anchor rope) paid out into the water; while this does not provide much drag, it can act as a drogue and aid in running downwind. In "The Sea-Wolf", author and sailor Jack London described using various torn spars and sails, tied to a line, as an improvised sea anchor. A sail, weighed down with an anchor chain or other heavy object, will also work as an improvised sea anchor.