Italics are used in a couple of different ways in this book.
First, italics are used when writing the name of a ship. For example, the author writes Andrea Gail in italics to alert readers that he is talking about the ship. This is a fairly standard grammatical format, and I don't think that this is what the question is asking about.
The other main use of italics in the book is for conversations that are taking place between people; however, italics are not used for every conversation. They are only used for conversations or communications that are happening over radio or telephone.
What's also key about the dialogue in this book is whether or not it has quotation marks around it. The author has stated that no dialogue was made up; however, some of the dialogue is based on people's recollections. If it's not in quotation marks, the dialogue does not represent word-for-word what that person actually said. If the dialogue is actually a verbatim quote, then quotation marks are used.