Fire crews were called to the incident just before midday on Saturday.
The blaze of the 85ft motorboat has sent thick plumes of dark smoke billowing into the air.
A spokesperson from Devon and Somerset Fire and Rescue Service said: “85ft motorboat on fire. Fire crews called at 11.57am.
“Three fire engines on the scene. Liaising with the harbourmaster.”
One eyewitness described hearing “explosions” as the fire broke out.
They told Devon Live: “It started off 15 minutes ago alongside pontoon.
“There were explosions.
“Ropes burnt through and it drifted into pier.
“Firemen took trolleys and pumps along pontoons but water not reaching.
“Some people are trapped on the other end of the pier and can’t get back.
“Firemen are running along the pier. Still huge flames.”
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