What do I have to do on the water to protest another boat?

If the protest is about an incident you saw in the racing area, hail "Protest" at the first reasonable opportunity. If your boat is longer than 6 metres, also conspicuously display a red flag at the first reasonable opportunity, and keep it displayed until you are no longer racing.

If the other boat is not within hailing distance, the incident was an error in sailing the course, or you did not see it in the racing area, you only need to inform the other boat of your intention to protest at the first reasonable opportunity.

Source: Racing Rules of Sailing 2025-2028 with Australian Sailing prescriptions, rule 60.2.