A warning light assembly for a watercraft or land vehicle includes a selectively activatable light for mounting to the watercraft or vehicle such that the light is clearly visible to nearby operators/drivers. There is a power source for being carried by the watercraft/vehicle. The light is operatively connected to the power source through a switch that is alternatable between open and closed states. At least one mechanism is provided for sensing when the watercraft/vehicle is decelerating. A control device is responsive to the sensing mechanism for closing the switch only when the watercraft/vehicle decelerates in a longitudinal direction and simultaneously there is no change in acceleration in a transverse direction. This causes the light to activate so that a deceleration warning is provided to nearby persons.