Middleton, with a big smile, poured champagne over the lifeboat after Prince William made a short statement asking that all those who used the boat be blessed.
Middleton looked elegant and comfortable in her new public role as a princess-to-be. She wore a three-quarter length fitted beige coat and had her long hair swept back. The look was topped by a chic feathery hat.
Prince William, a search and rescue helicopter pilot at a Royal Air Force base in Wales, stood by his fiancee as both chatted with local dignitaries and members of the public.
Jean Critchley, 63, said the wait was worth it for the chance to see the couple.
"They are a lovely couple. He is a great lad and she is a beautiful girl," she said after watching the event with her husband and grandson.
They plan to marry April 29 at Westminster Abbey with just under 2,000 guests attending.
Prince William named the Trearddur Bay Lifeboat Station's new lifeboat the "Hereford Endeavour" at Thursday's ceremony.
The couple will visit their alma mater, the University of St. Andrews in Scotland, on Friday.