Interestingly... Grand Princess is one of the few ships that does REAL weddings on board, but they also have a wedding chapel and I am sure they do renewals. Ask your travel agent if this can be arranged in advance. If not, I would bet you it can be arranged on board by contacting the front desk.
I have seen the wedding chapel, it is beautiful, in fact I was present in the chapel for the first wedding at sea in decades during this inaugural cruise, Note the Captain officiating, and the groom about to kiss the bride...
I took this pic myself :-)