Nicol Wood, 22, of Banff, admitted disturbing the school of bottlenose dolphins a mile offshore at Faw Bay, Macduff.
Dave MacKinnon, Force Wildlife Crime Officer for Grampian Police, said he hoped the conviction would send a warning to other jet-skiers.
He said: "The message that we want to get out is that this activity is an offence. These marine mammals are no different from other protected wildlife.
"They have to feed and look after their young. The activity of a jet-ski being driven recklessly around them is likely to have led to them being disturbed."
He added: "Wildlife crime investigation is not all about getting people to court and convictions, but education of the public and awareness raising.
"Hopefully the conclusion to this incident will send a strong message to people who use the marine environment for their work and leisure. What we ask is that people using such crafts as jet-skis and speed boats do so in a responsible manner for their safety and that of others, including protected wildlife."
Wood had previously denied the charge of intentionally or recklessly disturbing or harassing a dolphin by splashing water in an attempt to attract the attention of the creatures on 24 June last year.
The allegation that he intentionally disturbed the dolphins was later deleted and Wood was fined following an appearance at Banff Sheriff Court on Monday.
Some jet-skiers and power boaters have been caught tormenting dolphins by chasing and circling them.
However, this is the first time anyone has been successfully convicted of the offence. It follows an amendment under the Wildlife and Countryside Act 1981.
In the past, experts have expressed concern that breeding grounds are being disturbed by jet-skiers, with young calves separated from their mothers when chased off feeding grounds.
Anne Saunders, Scottish Projects Officer for the Marine Conservation Society Scotland, said: "We welcome the fact that harassment of marine wildlife has been recognised as an offence in practice and not just in theory.
"There is plenty room in our seas for recreation, provided marine wildlife is given adequate space: the sea is their natural habitat and we must respect that when using it.
"All sea users should be aware of and follow the Marine Wildlife Watching Code."
PRESSURE ON JET-SKIERS
JET-SKIERS have come under heavy scrutiny in recent years.
Loch Lomond is the main focus for attempts to restrict the sport, along with the use of speedboats.
Speed restrictions are in place in certain areas and the presence of police and ranger staff on the water has increased.