What is a tornado that forms over water called?

A tornado that forms over water is called a waterspout.

Waterspouts are rotating columns of air that extend from a cloud toward a body of water. Most are “fair-weather” waterspouts, which usually form beneath developing cumulus clouds in warm, humid conditions with relatively light winds. They are often weaker and shorter-lived than the tornadoes associated with severe supercell thunderstorms.

A common mix-up is assuming every waterspout is a separate phenomenon from a tornado. Meteorologists also recognize tornadic waterspouts: ordinary, mesocyclone-associated tornadoes that form over water or move from land onto water. The National Weather Service therefore commonly defines a waterspout simply as a tornado over water.

Waterspouts can occur on oceans, coastal waters, lakes and even the Great Lakes. Their visible funnel is made largely of condensed water vapor, while spray at the surface can create the distinctive ring often seen around the vortex.