There are two excellent web sites where you can get access to books that are in the public domain and, thus, can be fully accessed. The first is Project Gutenberg. Its developers have made hundreds of texts easily available to readers. Unfortunately, Christopher Columbus's journal is not one of those texts.
The other web site where you can find full texts of books that are in the public domain and excerpts from other books is books.google.com. I searched that site for you and found a couple of books that you might find interesting: First Voyage to America: From the Log of the Santa Maria and With the Admiral of the Ocean Sea: A Narrative of the First Voyage to the Western World. The first book is the actual diary of Columbus; the second is based on the diary.
I've pasted links to both of those books in the Sources section. I hope they help you!