How do you signal that you agree to let a boat pass on your port left side?

To signal your agreement for a boat to pass on your port (left) side, you can use a series of short blasts on your horn, typically two short blasts. Additionally, you should make sure to maintain your course and speed while ensuring your navigation lights are visible, indicating your position. It's also a good practice to make eye contact or use hand signals if possible, to confirm intentions with the other vessel.