Hi Susan,
Welcome!!!
Although I have not had a cruise wedding, or been to one, I booked an Eastern Caribbean cruise for January 2009, about two months ago. And I can tell you that they book up FAR in advance!! To have that many guests my advice would be to get on it now!!!!!!! When I booked our vacation cruise, they were actually booking as far as 2011 which I thought was outrageous because who knows what you will be doing in 2011!
From the research I did, the cruises stop at each port for less than a day. (sometimes 9-6, sometimes it is more like noon-6) so an onshore wedding would need to be timed JUST RIGHT. And the ships sail pretty much around dinner time, so you couldn't really do a dinner type thing on an island or you will get left behind.
I have heard (I briefly looked into it) that onboard weddings can be amazing...the ships have some private banquet type rooms, or perhaps you can reserve an area of an outside deck and i have read that the ships sail at sunset and it is really beautiful.
Not too sure about costs though.
Goodluck, hopefully you can chat with some other cruise brides to be, and there are some cruise wedding areas in this forum you should check out!
~Lori