I have seen it open during MM. I just checked the time stamp on a photo from last September and we rode at 7:43. We might have been the first ones on and had a whole boat to ourselves and couldn’t see anyone else the whole ride. So while it might not be open right at the beginning of MM it can open before official park opening.
ETA: should have looked more closely at the date, it was July not September! But we did get on the ride at 7:38.