A cylinder boot is a rubber or plastic cover for the bottom end of a diving cylinder

Diving cylinder

A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....

. It is intended to make it easier to stand the cylinder on end on land, and to avoid the metal cylinder damaging tiles during initial training in swimming pool

Swimming pool

A swimming pool, swimming bath, wading pool, or simply a pool, is a container filled with water intended for swimming or water-based recreation. There are many standard sizes; the largest is the Olympic-size swimming pool...

s.

Some accuse that cylinder boots hold wet in, risking corrosion

Corrosion

Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

 of the bottom end of the cylinder, and add to hydrodynamic drag when the diver swims. See Doing It Right#"Doing It Wrongly".