This sort of advice would be more useful 200 pages ago... but anyway it's always good to search for an annotated edition. I read Norton critical edition,* it was really good, had diagrams showing what the different parts of the boat are called, a glossary, supplementary essays, and throughout the text all sorts of footnotes (some of them maybe too explicitly interpretative, but oh well). But I believe even the slightly more modest but still seriously prepared editions such as Oxford World Classics would do the job.

* a critical edition means the editors didn't just reproduce an existing text, but worked off the most "original" materials available, such as the first edition or the author's own manuscripts

There are also a few, more recent companion books that give a lot of historical context to both Melville and whaling, etc

Why Read Moby Dick? By Nathaniel Philbrick

Ahab's Rolling Sea, by Richard J King

Cheers dickhead, online resources are key when youre reading those old nautical tales!

Here's a History Channel special about the story that inspired Moby Dick. I've never read Moby Dick, but there were things discovered in a journal well after the book came out that are pretty wild.