The couple was in Wales for the naming of a new lifeboat at a lifeboat station in Treardurr Bay.
The prince made a short statement asking that all those who used the boat be blessed, and then Middleton, with a big smile, poured champagne over the lifeboat.
The visit had been highly anticipated because the couple has been keeping a very low profile since announcing their engagement last November.
They've only made one other appearance together since then at a charity event.
The couple will visit their alma mater, the University of St. Andrews in Scotland on Friday.
They're getting married at Westminster Abbey with nearly 2,000 guests attending on April 29.
The Associated Press contributed to this report.