Endings
There are two different endings to the game, each just a slight variation of the other.
Ending 1
When you first climb the footholds behind the river fork on Monkey IslandTM, you'll encounter a "sculpture." If you leave this alone and climb up to the cliff top, you can push the boulder of the edge to catapult it across the island and into your ship. If you aleady moved the art to hit the tree, just Push it twice to set it back to its original position.
Once the ship is sunk, so are your crew. Don't worry, they're not dead...just very wet. Continue the game until you gain the Magic Seltzer Bottle. You return to the ghost ship to find it gone, and have a chat with Bob. Herman Toothrot will show up and escort everyone off the island. After you defeat LeChuck, you can trigger a scene while talking to Elaine that shows your crew still stranded on the island.
Ending 2
When you first climb the footholds behind the river fork on Monkey IslandTM, you'll encounter a "sculpture." Pull it twice and climb up to the cliff top, then you can push the boulder of the edge to catapult it across the island smash the banana tree on the beach. Don't do anything else with the catapult.
Continue the game until you gain the Magic Seltzer Bottle. You return to the ghost ship to find it gone, and have a chat with Bob. Your crew will show up and escort everyone off the island. After you defeat LeChuck, you can trigger a scene while talking to Elaine that shows Herman Toothrot still stranded on the island.