Haslett, who joined Celestyal in February 2023 from Virgin Atlantic, will leave the line in October. A search for his successor is under way.
Celestyal said Haslett had played a key role in its commercial transformation, leading on strategy across revenue management, sales, marketing, partnerships and distribution.
Haslett said the opportunity to join "a globally recognised sporting institution at such an exciting time in its journey" was one he could not pass up.
Chris Theophilides, Celestyal Chief Executive, said: “Over the past three-and-a-half years, Lee has made a significant contribution to Celestyal’s transformation journey, and I would like to thank him for everything he has brought to the business.
"His leadership, commercial insight and relentless focus on growth have helped strengthen our commercial organisation, with a strong team in place to continue the successful growth of the business.
"During his time with Celestyal, the company has relaunched its brand, welcomed two new vessels to the fleet and expanded its itineraries beyond Greece. We congratulate Lee on this exciting opportunity and wish him every success in the next stage of his career."
Haslett added: “It has been an incredible privilege to be part of Celestyal’s journey over the past three and a half years. I am immensely proud of what we have achieved together and grateful to Chris, the leadership team and all of my colleagues for their support, friendship and ambition.
"I leave with enormous confidence in Celestyal’s future and look forward to following its continued success.”
Haslett will remain with Celestyal until October "to support a smooth transition and help ensure the continued delivery of key strategic initiatives", said Celestyal.