Like some of the other checklists, the ditching checklist has deferred items on the "Descent Checklist" part. Since it takes the place of the normal descent checklist, they have to show something for an autobrake setting to fill in that spot to keep things the same. So "OFF" just take the place of whatever setting you might otherwise consider or reply to in normal operations.
In terms of system operations, I don't think that it makes a difference at all, if you were to select a brake setting.