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This is from "FEDERAL REQUIREMENTS FOR RECREATIONAL BOATS"

Ventilation
All vessels built after April 25, 1940 which use gasoline for electrical generation, mechanical power or propulsion are required to be equipped with a ventilation system.

A natural ventilation system consists of at least two ventilator ducts, fitted with cowls or their equivalent:

A minimum of one intake duct installed so as to extend from the open atmosphere to the lower portion of the bilge; and
A minimum of one exhaust duct installed so as to extend to a point at least midway to the bilge or at least below the level of the carburetor air intake.

A powered ventilation system consists of one or more exhaust blowers. Each intake duct for an exhaust blower should be in the owner one-third of the compartment and above the normal accumulation of bilge water.

Between April 25, 1950 and July 31. 1978, the regulations covering ventilation systems applied to the owner/operator. If your boat was built between April 25, 1940 and July 31, 1978, a natural ventilation system is required for all engine and fuel tank compartments, and other spaces to which explosive or flammable gases and vapors from these compartments may flow, except compartments which are open to the atmosphere. There was no requirement for a powered ventilation system; however, some boats were equipped with a blower.

The Coast Guard Ventilation Standard, a manufacturer requirement, applies to all boats built on or after August 1, 1980. Some builders began manufacturing boats in compliance with the Ventilation Standard as early as August 1978. If you boat was built on or after August 1, 1978 it might have been equipped with either (1) a natural ventilation system, or (2) both a natural ventilation system and a powered ventilation system. If your boat bears a label containing the words, "This boat complies with U.S. Coast Guard safety standards," etc., you can assume that the design of your boat’s ventilation system meets applicable regulations.

Boats build after August 1, 1980 which comply with the Coast Guard Ventilation Standard must display at each ignition switch, a label which contains following information.

Warning
Gasoline vapor can explode. Before starting engine operate blower for at least 4 minutes and check engine compartment bilge for gasoline vapor.

All owners are responsible for keeping their boats’ ventilation system in operating condition. This means making sure openings are free of obstructions, ducts are not blocked or torn` blowers are operating properly and worn out components are replaced with equivalent marine type equipment.

For the CME, all blower motors installed in exhaust ducks must be in working conditions regardless of the date manufacture.