The Curse of Monkey Island/The Demise of the Zombie Pirate LeChuck

The cannon gallery
When the opening video ends Guybrush is left standing in the cannon deck. Next to him is a short pirate firing a cannon. Since he's on the side of LeChuck, that needs to be stopped. Talking to the pirate reveals that he is Wally from the previous game. To stop him, simply inform him that he isn't cut out to be a pirate and he will break down into a sobbing mess.

Next up is addressing the landing party of skeletons. With Wally out of commission it's as simple as using the cannon yourself. This launches a little minigame where you get to aim and shoot the cannon. Move your mouse to aim and click to fire.

With the immediate concerns taken care off, it's time to break out of this place. Brute force will solve things, but you need a lot of it, far more than Guybrush himself can do. When Wally broke down he dropped his fake hook, pick it up. Also pick up the cannon loading rod from the wall. With the landing party sunk you can now squeeze past the cannon and peek out of the ship. There is some debris floating after the battle. Combine the fake hook and the loading rod and you will have the tool needed to fish the debris out of the water. Along with the debris you also collected a cutlass. Climb back inside and use the cutlass on the cannon restraint rope. With the rope removed all you now have to do is fire the cannon one last time.

The treasure hold
There is a lot of stuff in here, but Guybrush is only willing to pick up two things. The first is a bag of wooden nickels. And underneath the bag lies a diamond ring.

To escape use the diamond ring with the tiny porthole window.