There are a total of 10 diaries in Dead Island Riptide. Finding all of them is a requirement for the Hoarder trophy / achievement (find all 61 collectibles). In the inventory (press select) you can see which collectibles you have already found. They are all numbered, so you can identify missing collectibles easily. This video guide shows them in order of accessibility, but the time line below provides a detailed overview. After finding a collectible it will be saved automatically. You can still access all areas, side quests and collectibles until you use the boat in chapter 13.