How do I capture people on ships?
 

I have a couple of quests where I must capture some people aboard Navy ships. I've sunk multiple ships but the people are never captured. Where do I find which ships hold these people? Do I have to board these ships before sinking them? If so, how do I do that??

you just need to sink the right ship, your journal should say what it is, for example, EITC Frigate or Navy Galleon .........

As Lucy said, you have to sink the correct type of ship. On the Journal it will tell you "Capture Capt. Smith from a Navy Frigate and maroon him on a deserted island". Just sink frigates until it shows that he was captured. If it says to capture Capt. Smith from a Navy galleon, keep sinking galleons until he's captured.

Sometimes it takes a few ships to sink before you get your target. I guess theyre makin it more realistic....I mean how many Capt Montrose do you find on ships? He cant be on all at the same time right lol.