Beechcomber, you are what I was 12 months ago!!!.
My wife and I (42 and 43) cruised with our daughter and son (15 and 14) on Navigator of the Seas out of Barcelona last June, and it was far and away the best holiday we've ever had, to the extent that we're already booked to do the Baltic with Carnival in 2009 and the sprogs are keeping up a relentless campaign to cruise the Caribbean on the Oasis of the Seas in 2010.
Regarding your worry about the water, trust me when I say most of the time you forget you are on a ship. If you cruise out of Southampton, there are a few sea days taking in a Bay of Biscay at the start which I believe can be a little rough. With that in mind, a fly cruise which departs from a Med port might be a more suitable option if nervousness about the water is an issue, but believe me when I say there is little or no sense of movement
Two options with Royal Caribbean are either Navigator of the Seas, which cruises out of Civitavecchia (50km from Rome) next year alternating between an Eastern and Western Mediterranean itinerary, or Voyager of the Seas, which cruises a seven day Western Med itinerary out of Barcelona. Both ships are virtually identical, and having cruised Navigator this year (on the recommendation of previous posters in this thread it must be said), I would absolutely recommend either ship.