It's normally because you have decided to shoot nearly a dozen shots at me and my son sailing to an island.
My son and I were playing, and we just got done with an undersea treasury and were looking for an island to sell our stuff on. The first one had a galleon, I'm like "There is a whole lot of nope there."
2nd one was a sloop just starting out, and I decided to sail to the other side to not cause a fuss. The crew on the other ship decided to attack, well, I let them attack and miss grandly on us... not one shot was even close.
I sailed around the ship and decided to go to another island, they decided to let another volley of rounds at us. I got to the cannon, aimed a fire shot at them and let loose a single shot. It arced in the air and I smiled as I new it was going to hit their mast. I didn't expect much but they were carrying around a few explosive barrels, and all my son and I see are two random explosions... and a ship caught quickly on fire.
My guess was that they both died in the explosions and didn't come back until their ship was engulfed in flames, I just sailed past and let them deal with their actions.
My son asked me, "Why did you only shoot them once, why didn't you sink them?"
I said, "That was a learning shot."
He asked, "Learning shot?"
I said, "Yup, they learned to not shoot at someone just because they could."
He asked me then, "What happens if they follow?"
I said simply, "They learn what it means to be sunk."
We had a full round of ammo that could have put them to sleep, unable to repair their ships, dropped their anchor and raised their sails...
After we went to the Sovereign, we saw a ship just starting out, we had two crates for general supplies, I filled one up with our food, and wood. I put all the ammo we had and special shots into the 2nd one.
I dropped them off with my son on the ship and had my Mic on. I didn't see them on the ship, but I said as we were walking past the tavern, "We let something on your ship, have fun!"
My son asked me, "Why did you do that?"
I said, "Why not? We are leaving and it's better someone else can use it, than it being lost after we leave. That plus it's good to be nice sometimes."