There was a special on CNN last week and one segment was about cruise ship security. It showed divers in Miami examining the every inch of the underwater hulls of cruise ships. They do random dives so that no one knows when the inspections will be done.
It also said that background checks are done on every passenger.
I know that the divers were doing the inspections before the UK incident, but I imagine things have been beefed up since then.
Naturally, a ship could be attacked. So could a plane, a mall, our water supplies, etc. Hopefully your family will realize that we can't live in fear. I feel that taking a cruise is every bit as safe as any other form of vacation and ten times more fun!!
Go for the cruise and enjoy. You'll be glad you did!