One prolonged blast is a warning signal (for example, used when coming around a blind bend or exiting a slip). Five or more short rapid blasts signal danger, or signal that you do not understand or disagree with the other boater's intentions.
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Five Short Blasts - This is the DANGER signal. Remember, that when you approach another vessel and hear either one or two short blasts, and you both understand their signal and can safely let them do it, then you are required to respond with the same signal in response.
The general emergency alarm on the ship is recognised by 7 short ringings of the bell followed by a long ring or using the ship horn signal of 7 short blasts followed by 1 long blast.
Horns can warn boats away from ships, structures and areas where they might run aground. And mariners can use the sounds along with their charts to navigate when their view is impaired by fog, smoke and heavy rain.