Installing the app on a new device will have the passcode disabled by default but still load all previous journal entries. I share my Apple account with family members and installing Day One on one of their devices would gives them access to all my journals without requiring a passcode. I've switched to Dropbox for syncing to prevent this but I'm still bothered by the fact that journal entries are stored in plain text and extremely easy to access if someone has access to my computer and goes through my Dropbox folder.
I'm thinking about using OneNote with a password protected section since it has a decent iOS and Mac app, but I'm curious if there are better options. Those of you who keep personal journals with sensitive information, what do you use?