So i've never had this happen quite this quickly before. A skeleton sloop was hanging around near the island I was treasure hunting on solo, so once I finished I decided "sure, what the hell, why not?" and decided to engage it.
I ended up firing a single cannonball shot into it just below the water line, then realized I only had the single cannon ball on me. I was going to run over and grab some more but i still had a few firebombs, and it was close so i figured i'd toss them, then it dawned on me to load them into the cannon instead, so i fired three firebombs into it along it's length, pretty much setting the entire deck ablaze. At the same time they opened fire, hitting me a few times, and since i was close to the island I also had to dodge rocks, so I did that, went down and fixed holes real quick. Then grabbed a load of cannon balls, turned around, sailed back toward them, preparing to engage again only to catch up just in time to see them sinking... It doesn't get much easier than that (without cheesing them with kegs)
My guess is the flames kept them busy/damaged enough that they never had a chance to repair? Anyway, a singe cannon ball shot kill (without hitting a powderkeg) definitely has to be a record for me that I don't see myself beating any time soon :)