It is at its worst on balmy summer evenings. At first it sounds like the drone of a lawnmower, but the pitch is higher, the whine louder - like a chainsaw or a dentist's drill. The coast or shoreline may be a mile away but this is a jet ski at work.

Also known as "personal watercraft", jet skis are subject to certain restrictions since the introduction of safety legislation over a year ago.

Under 16-year-olds are not allowed to use them, nor can they be "mixed" with alcohol. Several local authorities have banned them, mainly due to complaints about noise and nuisance to other water users.

However, in North America, where the injury rate for use of jet skis is more than eight times higher than in motorboats, there is growing concern about the craft's impact on marine and freshwater environments.

The Bluewater Network environmental group cites the Park Service at Lake Mead in Nevada, which estimates that jet skis can release 27,000 gallons of gas and oil into the park's waters on a busy weekend.

This can have a severe impact on the shallow marine/freshwater environment in which jet skis operate.

The group says jet ski users "routinely harass waterfowl in their nesting areas, as well as marine mammals such as dolphins, seals, humpback whales and manatees".

The Irish Whale and Dolphin Group hasn't found them to be a major problem, but Dr Simon Berrow of the group says the biggest risk is their ability to change direction suddenly. He recalls Cork's harbour office had to take action over the antics of several water users during a visit of humpback whales to Cobh several years ago. Geoff Magee, who runs dolphin-watching trips on the Shannon Estuary from Carrigaholt, Co Clare, believes jet skis are a major problem and says operators should be required to take out licences.

"Just inside the village pier at Carrigaholt is a great place for children to swim, but it is not so great when small boats and jet skis belt in there at a rate of knots," he says. "Anything that moves fast and erratically around dolphin is also very stressful for the cetaceans. It is an issue of respect for other water users, and marine life." There have been, however, several positive developments. Sales of jet skis have fallen in recent months. David O'Brien of the Irish Marine Federation says EU emissions directives to protect the environment will require use of four-stroke marine engines from the year 2008. Many manufacturers have already started converting to four-stroke.

And local authorities - including Fingal in Dublin and Galway City Council - have consulted Irish Water Safety about introducing zoned areas, to permit restricted use, rather than an outright ban which could prove unworkable.

Significantly, only one case has come before the courts to date. A jet skier was fined 350 euro for breaching by-laws when he drove too close to a beach used by swimmers on Inis Oirr, the smallest of the Aran Islands.

A lifeguard, swimming instructor and a garda had witnessed what happened.

Probably there have been many more instances during the past year when the law was not enforced - begging the question as to whether education is better than legislation in the long term.