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You need to get in touch with 'theweddingexperience' this company does the weddings at sea for Princess and P&O, They would tell you all availability. I am getting married on Sea Princess in May this year but I booked this a few months ago, there is a lot of paperwork etc to get licences and such like, I can email you their wedding brochure of you want, just post your email on her.
Theweddingexperience.com handles weddings for RCI.
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. I think you have to book at least 3 months before, but details are all in the brochure.