A valve (1,1) for a marine propulsion unit comprises a body (2) having an internal passage (3) through the valve body (2). An aperture (5) in the valve body communicates with the internal passage (3). A piston (4,4) is moveably mounted within the internal passage. The valve is configured such that, upon immersion or expected immersion of part (such as an air intake) of an associated propulsion unit in water or tilting of the associated propulsion unit beyond a predefined angle, the piston moves to a first position within the internal passage in which it seals the aperture. If for example the air intake of a marine propulsion unit of a vessel is connected to the aperture (3), the effect of the piston sealing the aperture (5) is to isolate the air intake from the atmosphere. Thus if the vessel sinks or capsizes, the valve closes the air intake of the propulsion unit and so prevents entry of water into the propulsion unit.