One of the problems is that for those of us who hold upper level licenses, we have to know morse code and can send single letter codes via whistle or light. It is a requirement for us to be able to send and receive messages via flashing light.
Now for just about everyone else you don't have that requirement and the probability that the person on the other vessel knowing morse code is nil to none.
Thus the correct signal would be 5 short blasts on the horn, asking the other vessel:
What are You Doing!? but if that power boat was drifting, then he was the give way vessel and should have moved. And if he was at
anchor it would have been either 5 short blasts
or 1 short, 1 prolong, 1 short blasts on the horn. As in accordance with the Rules.