If it is a safety hazard a Sécurité broadcast to all vessels in XXX harbour on a working channel and at low power perhaps, at the very most. However since few boats in a marina will have the radio on there is not a lot of point there.
That deals with the responsibilities of a master to report a discharge from his vessel. The question was about the VHF procedure for a harbourmaster to promulgate information about a discharge.
A simple "All Stations, All Stations, All Stations" would suffice. Securité is for safety broadcasts and I don't think a fuel spill represents a danger to shipping.
Like Ballybug Woodlouse I do have some reservations about a Sécurité broadcast being justified unless the spill was considered a hazard to navigation, which if large enough it could well be.
The wording would be Sécurité - Sécurité - Sécurité - All stations - All stations - this is XXX harbour radio - XXX harbour radio - Navigational warning - Switch to channel nn.
Then on the working channel
Sécurité - Sécurité - Sécurité - All stations - All stations - this is XXX harbour radio - XXX harbour radio. A serious oil spill has been reported in the YYY area of the harbour. All vessels are requested to avoid this area. OUT
Perhaps the message should be adress to "All stations in XXX Harbour". It should be repeated at appropriate intervals.