One flare is probably not enough. The other ship's watchkeeper may have seen it out of the corner of his eye, but he will always want to have a better look. It is at that precise moment that the flare will choose to go out. A second, or even third flare should be immediately available and not stowed under an occupied bunk. It is not an instant reaction.
Some big vessels have very large turning circles and very, very few ships are able to stop quickly.
If the merchant ship's officer of the watch has problems, so does the yacht watchkeeper, especially if he is on his own. Those of us who have spent many hours in a cockpit keeping a watch will know only too well how easy it is not to perform to the high standards expected of the RYA examiner. Watchkeeping on a fine summer's day with the sun shining is one thing. The same thing on a foul night is quite another. We might be tired, bored, cold, possibly wet, even seasick and looking forward to turning in again.