Water police are patrolling Port Phillip Bay in a crackdown on boaters who anchor in shipping lanes.
Recreational boat users are being warned to steer clear of big ships after eight incidents since the start of snapper season last month.
Among the incidents was the sighting of two jet skis stopped in the Williamstown Channel.
On October 5, the Searoad Mersey was turning out of Webb Dock to enter the Williamstown Channel when the ship was contacted by Melbourne Water Police and told of the jet skis in its path.
Transport Safety Victoria said recreational vessels drift-fishing or anchored in and around shipping lanes were breaking the law and putting people in danger.
Maritime safety director Peter Corcoran said big ships could take 500 metres to turn and create a two-metre wave as they passed.
He said large ships had no brakes and could not steer around small boats. Ship pilots also had limited visibility.
Boats must not anchor, drift or fish in the Transit Only Zone (TOZ) in the north of the bay, and are strongly advised not to anchor or drift in the high shipping hazard zone from Hovell Pile in the south to the TOZ.
Boaters who anchor in shipping channels risk a $295 on-the-spot fine.
