Plumes of black smoke could be seen coming from a boat in St Peter Port's harbour.
The blaze spread to a second boat in the QEII Marina.
All emergency services attended late on Sunday afternoon (6 June) until the fire had been put out.
Firefighters stopped the flames spreading to any more vessels.
Dominic Jones, from Jersey, was nearby:
The man and woman onboard - who had been preparing to set off to Herm - reported hearing a 'bang' when they started the engine.
The woman had entered the water to escape the fire. The man had tried to put it out with a fire extinguisher before the emergency services arrived.
There were no reported injuries.
Following a vessel fire in the QEII marina this afternoon, power and water supplies to J pontoon are currently unavailable. Our maintenance team will be working to restore services as soon as possible and we apologise for any inconvenience caused.— Guernsey Harbours (@gsyharbours) June 6, 2021