'Mike' means "my vessel is stopped and making no way through the water" which would not be correct as you are at anchor. It implicitly implies that you are underway.
'Lima' means, "you should stop your vessel instantly"
'X-Ray' means, "stop carrying out your intentions and watch for my signals"
I cannot approach this conversation from a point other than as a professional mariner, so what I recommend would be what I feel most professionals would respond to. What sailors, or even harbor craft professionals, would respond to, I cannot say. Hopefully, others will respond to that.
I think 'Uniform', 'you are running into danger' is a single letter signal, along with 'Charlie', 'November', and 'Sierra', that would be recognized by the average mate on watch. I know I've never forgotten it's meaning, something I cannot say for all the rest of the single letter signals. Fortunately, I did not have to use 'Juliet'.
If 'Uniform' didn't do the trick, I suppose I would still fall back on the five short blasts of the danger signal. If I'm going to die, I figure to go down blowing, and that would probably be the last signal heard from me. The Coast Guard, in court, said it was inappropriate and for use only by vessels underway. This was convenient for the master of the other vessel, because he contended that he thought my ship was underway, even though our anchor and deck lights were on. I have a pet peeve with the cruise ships in this area, as they are usually lit up like a Phillipino water taxi, and you cannot make out their sidelights. Fortunately, they are never going anywhere quickly and bearing drift is enough to determine their relavence.
'Uniform' is the correct and probably best understood signal.
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