You can't just walk past the mountains.
One if by land, two if by...
But you can't swim.
You need to sail around to the northern half of Monkey Island.
Have you seen any boats?
On any shores?
There's a rowboat on the shore that you first landed on upon arrival to Monkey Island.
But to use it you have to have oars.
How can I get some oars?
Use the oars with the rowboat to set sail.
You can dock on any of the shores on the map screens.