CCGS Sir William Alexander is a Canadian Coast Guard Ship and is classed a "Light Icebreaker - Major Navaids Tender". She is currently assigned to CCG Maritimes Region and is homeported at CCG Base Dartmouth, in Dartmouth, Nova Scotia.
The Sir William Alexander participated in the mission as a supply vessel, and also to effect repairs to aids to navigation (navaids) and provide a more suitable staging platform for relief operations, given her high-capacity main hoist, as well as the ability to carry and stage "sea trucks" (similar to a small landing barge).
The decision to assign Sir William Alexander to the Operation UNISON task force was unprecedented in Canadian Coast Guard history as no icebreaker from the service has operated for an extended period of time in southern tropical waters such as the Gulf of Mexico, aside from transiting the Panama Canal to and from British Columbia. The icebreaker's engines are designed for operation in cold Arctic waters and she had operate at reduced speed to avoid overheating the further south she travels — the main reason arrived approximately one week after the navy ships leading the task force. Another possible reason for using the Sir William Alexander is that the navy's east coast Auxiliary Oil Replenishment (AOR) and supply vessel, HMCS Preserver, was unavailable due to complications resulting from an extended refit and repairs.
On September 19, 2005 it was announced that the three warships were no longer needed in the Gulf of Mexico, given the massive U.S. military response as well as increasing civilian aid flowing into the region. The Sir William Alexander was exempted from returning to Canada however, as her heavy lift capabilities were considered useful for ongoing repairs to aids to navigation which were destroyed by Hurricane Katrina, and later Hurricane Rita (September 23). On September 28, the Minister of Fisheries and Oceans announced that the Sir William Alexander was being retasked from relief efforts and navigation systems repair to assist the National Oceanic and Atmospheric Administration in repairing the damaged network of weather buoys along the Gulf of Mexico, Caribbean Sea, and Atlantic coasts. She returned to CCG Base Dartmouth from Operation UNISON on October 24, 2005.