wow what a great event !!! how about having your family and your parent's friends write down 49 reasons why you (and friends) love or admire or how your parents touched their life in a special way and then every night at dinner read 7 (save 1 on captain's formal night and maybe try to get the CD to seat them at the Captain's table for one of the evenings and have him read a reason (cruise related of course) or if you can't get seated at the captain's table perhaps he can read it at his cocktail party !
also how about presenting them with a cruise just for them for their 51st anniversary !!
no matter what you do I'm sure they will appreciate it and you have a great role model to stay married to celebrate your 50th