I am a registrar and I STRONGLY advise your daughter and her fiance to get a 100% assurance that the marriage will be legally recognised in the UK.
You will have agreat time my sister re newed her vows on this ship last year and it was lovely. The staff were great and took care of everything from flowers to cake p hotos were out of this world there was also a couple who got married and they had agreat day.
I recently got married at sea on another ship-Sea Princess- and had an absolutely wonderful wedding- The weddingexperience organised my wedding and I guess they will be organising your daughters so there will be no problem with marriage recognition in UK since they sort all that out for you and they need to be licensed to perform weddings which I am sure P&O are- check with whoever is organising the wedding for you. Anyway- weddings at sea are marvellous and I am sure the day will go off perfectly- wedding photos are wonderful with the sea in the background too! Passengers love a sea wedding and on the walkabout around the ship everyone says congratulations and claps as the bride comes past. As the bride, the one thing I would say is to tell your daughter to enjoy as much of the day as she can since it just passes so quickly, let others- i.e. the wedding coordinator- do everything for her and just let things go with the flow. I found the wedding staff to be absolutely amazing on the day from the make up person to the videographer- enjoy!