If your ArduPilot software is not set up for position hold, you will need to set up some parameters.
Log into BlueOS by connecting to the ROV and browsing to http://blueos.local http://192.168.2.2 -- this is the default address, if you have changed the ROV's IP address you will need to substitute it. You will see the following screen:
BlueOS landing page
Click the Autopilot Parameters tile. You should land on this page:
Autopilot Parameters Landing Page
Using the search and editing tools, change the parameter listed below to the given values. Don't forget to save the parameters after you change them.