Boats in Age of Water come in different shapes and Tiers.

Some have more cargo space, and some move faster. Usually, they are mutually exclusive.
Some boats in the same Tier also have better stats altogether than others.

For a comprehensive list of all boats and their stats, follow the link below (game wiki):

https://ageofwater.fandom.com/wiki/Hulls

If the link doesn't work, look for "age of water fandom wiki hulls" in your search engine.

To get better boats and unlock new hulls in your shipyard, you must disable, board, capture and bring NPC boats back to your base.

This guide will explain how to do these.

Step 1 should be to identify the ship, but since there are no tools to do so before boarding in the game, I'll skip it. You can refer to the link in the introduction to help you though.

How to disable a ship?

By killing its entire crew.

You need to disable/destroy the weapons stations and kill the helmsman.

The best way to do so is with Machine Guns and Flak Canons.

Other weapons such as the Canon and the Grenade Launcher can work too, but while they have a higher damage output, their fire rate can make it hard to hit, and you risk sinking the ship.

It would help if you also targeted the crew you need to kill instead of aiming at the hull or the red reticle.

Once the crew is disabled, the ship will stop, and a boarding prompt will appear above it.

Press and hold the "E" key by default to open the boarding screen.

In the boarding screen (Screenshot Soon™), you'll have your ship's name and status on the left, the boarding option in the centre, the name and status of the boarded ship on the right.

Usually, the boarded ship will have some issues:
-leak
-fire
-electricity

You can board the ship by dealing with the issue if you have the required tools in your cargo hold, or board and capture the ship directly with a boarding kit.

If you are out of boarding kit, you can receive them in daily rewards, or buy them in your base's shop for medallions.

You get medallions by participating in PVP events (caravan and pearls mining), or by defeating PVE bosses (Hoverboat and Quadrocopter).

Once you have captured the ship, you can transfer your cargo hold from your previous ship to your newly captured ship.

Congrats, you have a new ship!

1-Once you have boarded or captured a ship, the hull you will leave behind will remain in place for 20 minutes.

After that, it will be lost forever.

That includes what is in the cargo hold, so be very careful and transfer everything you can to the ship you plan to use after boarding.

EDIT: In a recent patch, when the hull disappear, the cargo content will become a lost loot crate, meaning you will be able to go back and fetch it.

1.5-Be careful if you move from a high cargo capacity ship to a torpedo/long/patrol boat, they have less cargo capacity, and you could sink it when transferring your cargo.

2-If you use the captured ship, it will be stripped of everything on deck, you will only have a wheel to move it.

That means, don't board and capture a ship in a danger zone or in the middle of a battle, you will be sunk.

The ship will still be damaged, keep that in mind.

You'll also need to place the weapons, structures, etc... by entering the build menu.

If you want to be able to build your new ship later on, you'll need to bring the ship back to your base, and have the naval engineer.

He will notify you that the new hull is available for construction.

I advise to use the teleporter to go back to the Three Whales faster, and to use your pirate/rubber boat to fetch your other hull.

You never lose your rubber boat, so you can safely abandon it to board and capture disabled ships.

I think that's it.

Ask in comments if you need more precision, or if I missed something.