As I understand it, it tends to be either the ship’s safe, the crew’s ransom, or, occasionally, takings from one or two containers.
I’m willing to let the aspect of ship height go. I suspect that the RPGs are the better option for pirates. Just threaten to destory the bridge, (or the rudder if the ship is not fully burdened), and then to begin working on the crew compartments while shooting anyone who shows their face over the rail. The threat is probably enough for most ships, who will then hang a ladder over the side to allow the pirates to board.
Here are two separate incidents of attacks on tankers, successful and unsuccessful. In the successful attack, the target was a 6300 ton tanker which would have had no significant freeboard since it was loaded. In the unsuccesful attack, the pirates were apparently unable to seriously damage the hull (and either ran out of rockets, had bad aim, or did not think to target the bridge and superstructure), and the high-riding 150,000 ton tanker pretty much ignored them.
The French cruise ship Le Ponant was siezed in January, but, as you can see, it is both susceptible to threats against its passengers, cabins, and rigging and it is not that high out of the water. (We are not talking about the QM 2, here.)
There appear to have been only two cruise ship attacks all together, the other occurring November 5, 2005.
The 1988 attack on the City of Poros is sometimes lumped with pirate attacks, but it was a terrorist operation where the attackers boarded as passengers.
By comparison, I was amazed to read in Science News some time ago that 10,000 shipping containers are lost overboard every year. If that number is accurate, the loss incurred by a few pirates is hardly significant. Before hiring mercenary marines, a ship operator might better invest in stronger tie downs and/or better crew training and inspection.
Plus pirates by and large don’t take containers, as I said. The “10,000” per year is disputed. The TT Club which insure most of the world’s container fleet says 2000 a year.
Lashing design is a compromise between strength and speed of lash/unlash. Plus it doesn’t matter how strongly you lash the container strongpoints if the strongpoints themselves rip out. Which they will if green water comes over the deck.
That is the problem with having all that unrtrained water in the oceans. Seasoned water knows to stay below the ship where it belongs, but you get that green, untrained water out in the open seas and, right away, it wants to run up on deck.
In addition to the cost/benefit analysis discussed above, I would think that sailors might be less interested in working for you when they find out that one of their job duties is getting into shootouts with pirates. Keep in mind that if the shipowner escalates by having people try to shoot the boarders, the pirates will quickly respond in kind by having their own guys with M-16s trying to shoot your guys with M-16s, and so forth.
As others have mentioned, your employees probably don’t consider protecting your cargo to be a cause worth getting shot over.