Well, during the "Manditory villainous explanation" near the end of Curse, LeChuck claims that there are catacombs that run under the sea-floor between Dinky and Monkey Island.  Although Guybrush started looking for Big Whoop on Dinky, he unbeknownstly (is that even a word?) went through the catacombs and ended up on Monkey Island.  Simple and convenient. -  June 26, 2:21 PM