Having a small, quick, fast ship, ensures two things:

1) You evade broadsides easily.
2) You outrun enemies easily.

But at a risk, they are weak and can't take much damage. You need to take care using those, they need skill, plus wind usage and navigation intuition. With triple hammocks upgrade, they can even house more crew, so you can get better swordfight sessions, even!

Royal sloops are a good ship even for late game. They can be found as pirate hunter vessels from Spanish enemy ports, or peace-treaty/ultimatum escort from Spanish governor to other nation!

Mail Runners, is usually the boat sent from the governor to deliver ultimatum or peace proposal from NON-Spanish governors; you simply attack and capture.

Mail Runners are great, it is said to be one of the fastest, if not the fastest ship (pinnace class ships are all contenders for this title) in the game and fully upgraded, carries enough men for any fight.

Using those also make your fleet sail faster, wasting less months at sea and navigating faster this vast region, too. Both can be great even for endgame. Best combo is said to be one of those two, combined with one member of the Frigate ship class, which has many cannons and room for cargo. So if you need a good firefight, just grab a Frigate variation and keep both.

In my brand new gog playthrough of the game, near the end of my career, i have a fully upgraded Mail Runner and a fully upgraded Ship of the Line, too. Until i found them, i went through the bulk of the adventure with a fully upgraded Pinnace (which could hold 90 men, too!) and Morgan's fully upgraded Large Frigate, an excellent ship (you HAVE to capture it)! I kept save so i can try to continue the adventure as long as possible, of course.

Try those and see for yourself! Pirating is quick, easy, fast and risky, just like those two beauties in title!

The game is too easy with a fully upgraded Royal Sloop. I have never actually used a Mail Runner, mostly because it involves betraying the nation that I am trying to help in the first place, and I can usually make more money by fighting and selling the captured privateers that go after the mail runner. But a fully upgraded pinnace or war canoe can wreak havoc on any other ship, so long as you can evade a broadside.

I won't take any ship into battle unless it has one of the maneuverability upgrades (copper plating or cotton sails). With both of those, you can make a brig dance around the largest broadsides.