Waterspouts are often described as tornadoes over water. The comparison holds for one type and misses the process that produces most of them.
Two different phenomena share the name
Tornadic waterspouts are true tornadoes that formed within a severe thunderstorm and happen to be over water, and they behave like their land counterparts.
Fair weather waterspouts develop under building cumulus clouds without a severe storm present, and they are far more common in most waterspout-prone regions.
The two look broadly similar from a distance, which is why the distinction is often lost in casual description and in photographs.
Fair weather spouts build from the water upward
The process begins with a boundary at the water surface where converging air is already rotating weakly in the horizontal plane.
A growing cumulus cloud overhead provides updraft, which stretches that rotation vertically and concentrates it into a narrow, faster-spinning column.
This is the reverse of the supercell process, where rotation develops within the storm aloft and works downward toward the surface.
Warm water and light wind favor them
Fair weather spouts are most common where water is warm relative to the air above it, which drives the instability the cumulus needs.
Light winds aloft help, since strong shear would tear apart a circulation that depends on a relatively undisturbed vertical column.
The Florida Keys and the Gulf coast see them regularly for these reasons, and the Great Lakes produce them in late summer and autumn.
They weaken quickly at the shoreline
A fair weather spout depends on the smooth warm surface beneath it, and moving over land introduces friction and different surface heating.
The circulation typically dissipates within a short distance inland, though it can produce damage comparable to a weak tornado while it lasts.
A tornadic waterspout carries no such limitation, since the parent storm sustains it and it continues as an ordinary tornado after landfall.
Warning practice reflects the difference
Forecast offices issue special marine warnings for waterspouts affecting coastal waters, aimed at boaters rather than at communities ashore.
A tornado warning is issued when a waterspout is expected to move onshore or when the parent storm is producing tornadic rotation.
Because fair weather spouts form quickly and are often seen before radar resolves them, visual reports from mariners remain an important input.
Forecasters also watch for the surface convergence lines and building cumulus fields that favor them, which allows a general statement of risk even when no individual spout can be predicted.