The sound signal that indicates to another boater to pass on your port side is one short blast of the horn. This is a standard maritime communication used to convey intentions between vessels. When you hear one short blast, it means the other vessel intends to pass you on its left side.
One short blast of the horn tells another boater that you wish them to pass on your left side.
on short blast
one short blast
2 short blasts
one short blast
one short blast
Two short blasts
Two short blasts
two short blasts
Two short blasts
one short blast
one short blast