Garry Luff, 36, of Waterlooville, had been stopped by officers from the Ministry of Defence Police Marine Unit after they recorded him driving his high performance speedboat at up to four times the harbour speed limit.
Magistrates at Portsmouth Magistrates’ Court fined him £800 for the crime.
Sergeant Alex Stypulkowski of the MOD Police Marine Unit said: ‘We had to act quickly with officers responding immediately to intercept the speeding craft, which was endangering both its occupants and anyone else nearby.
‘Whilst we always seek to educate as well as to enforce the law, this instance required a robust stance, and I am pleased to see the court imposing a significant fine of this nature.’
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Portsmouth Harbour, as well as the seafront up to half-a-mile off shore, is subject to a 10-knot speed limit.
The area is patrolled 24/7 by MoD police officers.