I did not see any mention of a floating line.
Tidal range looks to be about 12 feet, I have no idea what the tidal streams are like in the area, but at low water
, the buoy could be some distance from its anchor
, with the mooring
line at a shallow angle, easy to pick up in the prop with a boat passing between buoy and anchor
The person in charge of the boat should really have taken a look at what the tide was going to do when they elected to remain onboard, like Guy said, first course of action, get the missus over the side to cut the line