I called the customer care. they don't have any policy to cover Cruise ships and there is no option to add a rider to cover Cruises, even if I pay for it.
Hence I will be forced to take another policy of a different insurance company.
They provide four options for Cruises
US $50,000, with US $1000 for cruises
US $150,000, with US $1500 for cruises
US $250,000, with US $2000 for cruises
US $600,000, with US $2500 for cruises.
I was thinking of taking USD 250000 hoping that USD 2000 for Cruises will be adequate ONLY to allow me to board at the Carnival checkin at Sydney.