24th January, 2012 : Worth a read of this extensive FAQ for details on the restrictions in place during the Olympic events being held in Portland Harbour.
Of particular interest:-
Q. Can I dive in Portland Harbour and Weymouth Bay during the Games?
A. Yes but not in prohibited or restricted areas as shown on current Portland Diving Permits for 2012, additionally, during Race hours (R) diving cannot take place in any of the following areas: Competition area, Approach Sector, Approach Area and Access Route. After race hours (R) these areas will be reopened for general use. The Olympic Marina will be closed to divers throughout the Olympic HRO period (H).
See the FAQ for the definition of the above areas