What do two short blasts of a horn mean?

In boating, two short blasts of a horn indicate one boater's intention to pass another boat starboard-to-starboard, according to Boating Basics Online. This signal is used when it is impossible to pass port-to-port.

When a boater gives two short horn blasts signalling his intention to pass starboard-to-starboard, the other boater should reply with two short horn blasts to indicate he understands and is prepared for the move. It is preferable to pass port-to-port, but it is acceptable to pass starboard-to-starboard when other vessels or obstructions block the way, as long as other boaters understand what is about to occur. The signal for passing port-to-port is one short horn blast.