Flames engulfed a portable building and spread to another and a 14ft boat.
Fire crews were called to the blaze in Grove Ferry Road, Wickhambreaux, near Canterbury, just after 4.30am today.
They found one portable building well alight, and despite their efforts, it was completely destroyed.
The flames also spread to a neighbouring portable building and the motor cruiser, but firefighters managed to get it under control.
They used breathing apparatus, a hose reel, a compressed air foam system and a thermal imaging camera to put out the fire.
They then spent time damping down the area before leaving the scene just after 5.30am.
The cause of the fire is believed to be accidental.