As far as I am aware most of the modern cruise ships have flat bottoms and don't lie particularly low in the water and, yes this does enable them to get into shallower ports and doesn't help with the problem of rocking. Venice, for example, has a very shallow entrance and there is only a few metres clearance for the larger ships. If you want to see more I suggest you watch out for the occasional transmission on the freeview channels of modern ship building, there was recently one about the construction of Independence of the Seas. She has a flat bottom (which is more than can be said for some of the passengers it carries!)

You surely had the ideal opportunity to ask P&O technical staff any questions that you wished to ask when you were onboard the Ventura? I am sure that the Chief Engineer would have been happy to answer any questions that you had at the time? Happy cruising.