Fire engines from Brixham and another two from Paignton were called to Brixham Marina just after 6.30am.
On arrival crews found a fire in the bedroom of the 54-ft vessel involving a mattress.
Afterwards four people were checked over by an ambulance crew.
A force spokesman said: "This was a fire involving the bed and mattress in the cabin of the 54 motor yacht, 100% destroyed by heat and fire.
"The cabin sustained 50% damage by heat and smoke.
"The fire was extinguished using one hose reel jet, one main jet, one light portable pump, two breathing apparatus wearers and two thermal imaging cameras."