Fire crews from Ryde and Newport are attending the incident on the Esplanade alongside Bembridge Coastguard Rescue Team and Ryde Inshore Rescue.
Emergency services were alerted at around 13:30.
Firefighters are currently working to establish a mobile pump to extinguish the fire using sea water.
Island Echo understands the vessel is a visitor and that an adult and 2 children have escaped uninjured.
UPDATE @ 14:05 – Fire crews wearing breathing apparatus have taken position on the pontoon and are tackling the blaze with a main jet.
No other vessels are thought to be effected, but the pontoon is likely to be severely damaged.
A cordon has been erected as the multi-agency operation continues.
UPDATE @ 14:25 – Concerns are growing about the risk of pollution, with an estimated 300 litres of diesel on board the burning vessel.
Ryde Inshore Rescue are transporting firefighters out to into the harbour to tackle the flames from the West side.
UPDATE @ 14:42 – It has been confirmed that the vessel is a 35ft ‘Sealine’ motor cruiser and that the alarm was raised via VHF Channel 16 – the emergency channel for mariners – at 13:30.
A spokesperson for the Maritime and Coastguard Agency (MCA) has said:
“We are advising members of the public to keep at a safe distance whilst the emergency services get the fire under control.
“We have received reports that the person on board has made it safely off the boat and is ashore. He is safe and well and does not require medical treatment.
“This is an ongoing incident and we do not have any further information at this time”.
UPDATE @ 15:00 – The casualty vessel, which has broke free from one of her moorings and has spun 90 degrees, has now sunk in Ryde Harbour.
It remains unclear as to what caused the fire to develop, but it is thought to have centered around a tender attached to the rear of the boat.
A counter pollution team are en-route to Ryde to erect booms around the sunken boat in a bid to contain the diesel, which is now spreading across the harbour.
Ryde Harbour has been closed to all traffic and a notice has been issued to the Queen’s Harbour Master in Portsmouth.
UPDATE @ 16:58 – Members of the public are being advised to stay away from the immediate area of Ryde Harbour this evening.
Council staff are on site to advise the public to keep away from the area of the harbour, both immediately onshore and offshore. Harbour users are also being notified.
Anti-pollution booms are being put in place across the entrance to Ryde Harbour and spill kits are being utilised within the harbour walls. A strong smell of diesel lingers in the air.
The Isle of Wight Council is working with the owner of the boat to have it removed from the harbour area.