Why do i only see ocean when i load up TheIsland?
By default the editor only loads the TheIsland persistent level to save ressources. You need to load the sublevels you want to see/edit through the "Levels" tab. If you don't see the "Levels" tab go to "Window" in the main menu bar and tick "Levels". That should bring up the tab. You can then load the required sublevels by double-clicking on the name.
How do i update the Devkit without the Epic Games Launcher(Github and Content Diff)?
Updates are usually provided first through github(binaries) and a content diff archive. To update you need BOTH the binaries from github and the content diff package and extract them over your Devkit install.