I dunno! I don't remember how you find out that the Mad Monkey figurehead is a thing you want. Doesn't the dude in the antiques shop tell you about it, and tell you that it's on a shipwreck? Once you know that, it's not too obscure to go along to the library, repository of all knowledge (I think someone says that you can find out anything at the library or something, maybe Guybrush himself says it in conversation with the librarian), and look for a shipwreck related book. Being called Great Shipwrecks of Our Century, it immediately stands out if you know you're looking for the coordinates of a particular ship.
I certainly didn't have to check out every book, although looking through the entire card catalogue is still a bit annoying if you don't have the foresight to just look under S for Shipwrecks. It's probably there under W for Wrecks as well though, I can't be arsed checking World of MI's excellent library catalogue wiki thing to find out. Actually yeah I can, it would have taken me less time than it took to type out this paragraph. It's not under Wrecks but it is under Disasters (not the go-to place in the index, I wouldn't have thought) and also cross-referenced under Shipwrecks.
So now you know, man. That's if you ever come back, which you won't.
NOW YOU KNOW.