A 35ft cabin cruiser was engulfed by flames and sank into Burraneer Bay this afternoon.
Resident Don Widenstrom took these dramatic photos as the blaze took hold.
"I heard the boat was getting its batteries charged at the Burraneer Bay Marina around midday when it started to spark and caught fire," Mr Widenstrom said.
"Someone took it out into the middle of the bay and left it at the mooring. I saw a lot of smoke coming out of the cabin.
"The fire just grew larger and larger and burnt to the waterline about 2pm.
"The marina took some firies out on a boat and they had a pump they used to spray seawater on the fire. Once they arrived it took about ten minutes to extinguish the blaze."
A Fire and Rescue NSW spokesman said triple-O calls started coming through around 12.45pm saying a boat was on fire at the marina.
Fire crews from Cronulla and Miranda were called to the scene along with two Hazmat trucks from Alexandria and the Kogarah Duty Commander.
"A boat from the marina was about to take fire crews out to the boat with a portable pump," the spokesman said.
"A Marine Rescue was also in the area and attended with a second pump to assist.
"The boat sank about 2.30pmn and the fuel tank came loose and was towed back to shore.
"Fortunately there was no spill. About 200-litre has been decanted into Hazmat bins."
Investigations are continuing.