I was under the impression that the most common and nearly 100% effective anti-piracy weapon aboard cruise ships was called the engines. As in, 1) Don't sail close to pirate waters, and 2) if The Pirates: Band of Misfits, Jack Sparrow, or Eric Cartman and the Somalian Pirates WE! chase after you, FLOOR IT, and outrun their little SeaRays.

Of course, pouring the contents of the ship's septic system over the side on top of any would-be boarders would also be highly effective.

As would the simply locking any doors that are close enough to the waterline for pirate access, and removing exterior ladders that would allow them to climb from the waterline up to the lowest exterior deck (which can be anywhere from 20' to 60' above the waterline on a big ship).

A simple signal flare fired into the middle of their little boat would not sink them, but would blind them, burn them, and probably set their boat on fire; that's a good trick.

Or, maybe cruise ships are simply carrying a few actual guns now and can use them to ward off pirate attacks.

But, hey, I'm just spitballing here!