Is this Alan Dean Foster's Damned trilogy (found as the answer to previous story-ID questions here and here)? From Wikipedia's summary (emphasis mine, some bits removed):
However, most sentient species in the galaxy have evolved to be incapable of committing violence against other sentients (violence of any sort being most un-civilized, but against another sentient being an unthinkable crime), which leaves a shortage of warriors on both sides.
On a mission to find new resources and allies, a Weave scout ship discovers Earth circa late 20th/early 21st century AD and finds that humans are uniquely suited as allies, in that they have the ability to fight.
After the Weave scouts convince volunteers (mercenaries) from Earth to join the war, the tide turns for the Weave. Throughout the book the humans are greatly feared by the rest of the Weave, because of the human race's violent tendencies (the rest of the galaxy's species lived in harmony amongst themselves before they developed enough to reach out into space).
The human on his yacht in the Caribbean is described in this overview:
Will Dulac was a New Orleans composer who thought the tiny reef off Belize would be the perfect spot to drop anchor and finish his latest symphony in solitude. What he found instead was a group of alien visitors — a scouting party for the Weave, looking. for allies among what they believed to be a uniquely warlike race: Humans.
The second book in the series is called *The False Mirror", so there is indeed a mirror in the title as you thought.
This idea is used not just in a single series, but in so many that it has
its own page at TV Tropes, entitled "Humans are Warriors". Specifically, see the second version of this trope:
The second form of this trope makes humans useful to other aliens. Maybe the 'good' aliens have been fighting a losing war against the 'evil' aliens due to psychological limitations, numbers, or lack of training in the art of war. Maybe they are evenly matched or even winning against the enemy but want someone else to get shot at for a change. By working together with humans, they can actually put their advanced technology (or magic) to use effectively; in turn, humans gain the peacetime benefits of the aliens' advanced technology. Other aliens may be less scrupulous about the relationship, considering us less 'valuable partners' and more 'Battle Thralls'.
Many examples of this form of the trope are described on the linked TV Tropes page (scroll down a bit and click on 'Literature' under the heading for the second form of the trope), so if the Damned trilogy isn't what you're looking for, you might still be able to find it there.
