Harbour Master Captain David Barker said it was important to be mindful of others when enjoying local waters.
“Operators of personal watercraft, particularly high-performance craft such as jet skis, are asked to behave responsibly within our waters,” he said.
“In particular, jet skis and other high-performance craft must observe the six-knot speed limit in the island’s bays and should consider what effect they are having on marine wildlife and swimmers, especially in bays and near beaches.
“Please make safety a priority to avoid becoming a risk to yourself and to others.”
Speed limits from four to six knots are in force in St Peter Port and St Sampson’s harbours.
There is also a six knot speed limit in the approaches to Herm and Jethou.
It is a legal requirement that any vessel or craft capable of speeds more than 17 knots, including jet skis, are registered with Guernsey Harbours.
People should also be careful around designated Ramsar sites at Lihou Island and L’Eree headland and Herm, Jethou and The Humps.
Yellow buoys showing the speed limits are also being used to protect the breeding population of puffins around the cliffs of Herm.