Loiter is implemented and working and you can put a boat into hold position. When you say differential mode I think we refer to it as skid steering. The new Rover BETA has changed how it operates and makes it much easier. Information can be found at the bottom of these release notes
https://discuss.ardupilot.org/t/rover-3-2-0-rc1-is-available-for-beta-testing/20389
Thanks, Grant.