Luke English, who runs the RedBoats scuba diving business in Portsea, was at the pier at the time the rangers responded and offered to give them a ride to the possum on his boat.
"That was the first and last possum I will ever see on a mooring in Portsea," English told 9 News. "It was hilarious."
The possum became spooked when the vessel approached and jumped into the water, but the rangers were able to ensnare the animal with a net.
The marsupial was examined by a veterinarian and released back into the wild -- on dry land -- in the evening.
English said the possum had likely been nesting on a boat and panicked when the vessel started to move, ending up in the water.