Three boats were destroyed overnight in the third suspect boatyard fire in Mandurah in the past two months.
Fire crews from three stations were called to Estuary Marine on Paton Road, Greenfields about 1.40am.
They arrived to find three 5.5m boats on fire and more vessels under threat.
Firefighters moved several boats which were close to the flames before extinguishing the blaze.
The damage bill has been estimated at $100,000 and the cause deemed deliberate.
The owner of Estuary Marine, who declined to be named, said an unexploded Molotov cocktail was found under a boat this morning.
"Three boats are completely destroyed and one other boat beside has pretty substantial damage," he said.
"It's strange that we've had fires likes this at three different boat yards in the last few weeks.
"We definitely feel there's a connection, we'll let the police work that out."
He said there wasn't much he could do to protect his business against late night firebombing.
"It's not a nice feeling because you can't control what happens after hours," he said.
"This could happen to any business. I guess someone doesn't like boating."
Last week in Mandurah, seven boats were destroyed in a fire at Yunderup Marine on Pinjarra Road.
The blaze started about 11.30pm on Wednesday, March 13 and caused more than $300,000 damage.
Just weeks earlier, two 7.5m boats were set alight at Mandurah Aluminium and Marine Fabrication on Quarry Way, Greenfields.
The fire was reported about 12.30am on Friday, February 1 and caused $150,000 damage.
Police Arson Squad detectives are investigating all three incidents.
Anyone with information is urged to call Crime Stoppers on 1800 333 000.