Guernsey Ports have permitted people to dive in the area of the Pierheads and to the west of a line drawn from the old lifeboat slip to the turning dolphin on the New Jetty.
Diving is not allowed on any of the commercial berths to the east of the New Jetty.
Harbourmaster Captain David Barker explained: 'Christmas Day is the only day in which diving by organised groups can be carried out safely in the designated harbour areas.'
It is permitted on this day as there are very few scheduled commercial vessel traffic movements on Christmas Day.
This has become something of a festive tradition in Guernsey and a Gallo Roman shipwreck was discovered in the entrance to St Peter Port harbour during a previous Christmas Day dive.
'I would like to thank our staff who will be providing cover for the hyperbaric chamber at the Princess Elizabeth Hospital should the need be required,' Captain Barker added.
'For safety reasons we do insist that diving is carried out as part of an organised group and under prior permission given by Guernsey Ports.'