I really enjoy reading, and those deck 4 chairs are THE BEST!
Not sure what I'll take, since I don't cruise until December. But for me, since I read fast, I try to take a longer, more involved book. If I take a "beach book" (i.e. light reading), I'll finish it way too soon.
First cruise, I took Shelters of Stone (last book available in the Clan of the Cave Bear series), by Jean Auel. That lasted me through both flights and about 4 hours of reading on the cruise.
Other good books that would be relatively long reads would be the Tom Clancy books (military/spy fiction), Diana Gabaldon books (historic romance), Robert Jordan books (science fiction), Anne McCafferty's trilogy books (fantasy/science fiction).
Enjoy!!!