Apparently dropping sand from the aft of your boat acts as a good deterrent:
“sprinkling sand into the water around the rudder confuses the orca’s sonar” during an interaction, the Cruising Association has said.
“The orcas are coming in on sonar, not sight, we know that,” said John Burbeck, the Orca Project team leader at the Cruising Association. “And a few kilogrammes of sand around the rudder confuses that sonar picture they’re picking up.”