We got married on oriana just before christmas, and they did a realy good job. It was a lovely day and everything was so well organised we just had to turn up on time. Look on the P&O website under"cruised before / celebration cruises" for their downloadable brochure and price list. You can make it a big or small as you want which obviously affects the price. I think you have to book at least 3 months before, but details are all in the brochure. http://www.pocruises.com/pocruising/weddings-at-s ea.aspx.
We are getting married on Sea Princess in May and theweddingexperience.com organised it all. The have different packages depending what you want, but its not that expensive- my package cost 2,800 dollars and includes flowers, cake, champagne, photo package,lisence et.