Langstone Harbour Board has revealed 20 people were hauled before magistrates last year to be fined – and there could be a £850 bill waiting for speeders this summer.
Harbourmaster Captain Nigel Jardine said the 10 knot limit is often flouted and dangerous riders put people at risk.
Jet-skis are capable of speeds up to 70mph.
He said: 'It was getting dangerous to other vessels and children on the beach.
'The purpose of the speed limit is safety.
'We have got a multi-functional harbour – we have got ships coming in and fishermen and people bathing.'
The harbour has about 300 registered jet-ski users, and the number of people launching craft at Langstone reaches its peak during the holidays.
Capt Jardine said the majority of riders behave, but a minority risk injury to themselves or others.
He said: 'In previous years there have been accidents involving jet-skis.
'In some cases, riders have collided with other vessels and then driven off.'
There is already a designated zone south of Eastney Point where jet-skis can be driven at higher speeds, but Capt Jardine believes offenders prefer to remain in the harbour.
'They probably do not want to go out to the zone,' he said.
'What they want to do is stay in the harbour and show-off to their friends in the pub.'
A High Court ruling means speeding jet-skiers could also face Asbos, which the Harbour Board supports.
Capt Jardine has warned rogue riders to beware the harbour's strict policy towards speeders.
He said: 'We will patrol the harbour in the evenings, so those that used to speed can no longer be so sanguine about flouting our byelaws.
'We prosecuted 20 byelaw offenders last year, which shows we are serious – so beware.'
HAYLING HAZARDIn September, The News reported that high-powered jet-skis and motor boats had sparked fears among swimmers off Hayling Island.
Bathers said they were risking life and limb while using a 500-yard swimming-only stretch set up by Havant Borough Council. Swimmer Philip Newman, 50, said he had had near misses when craft crossed a line of marker buoys installed in 2006.
The council said it was difficult to patrol the area because it did not have a boat to pursue offenders.
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