We have had to use
trip insurance twice during cruises: One time we had to cancel and one time we had to leave ship early due to a death in the family at home. Both times the insurance company treated us well and backed everything: Unused cruise, Unused flights, new flights, rental cars, hotel rooms, taxis, food,--just keep your receipts!
Remember that your personal health insurance may not cover everything that happens overseas. If you are evacuated by helicopter from the ship, I have heard the cost could be in the tens of thousands of dollars!
What ever insurance you get, make sure it includes not only trip cancellation, but trip interruption as well. Remember if you miss the ship and booked your own air, you are responsible for getting to the next port to get on the ship. Trip insurance with an interruption clause will pay for the extras to get to the ship.