Massad Boulos traveled to Libya in July as the State Department’s senior Africa adviser. But as he talked to energy executives and government leaders, his other position was no less important. He was also the father-in-law of President Trump’s daughter Tiffany.

That family connection was so significant that some Libyan officials had privately taken to calling him “Abu Tiffany,” Arabic for “Tiffany’s father.”

While Mr. Boulos posed for photographs and announced deals to ramp up Libyan oil and gas production, Tiffany Trump and her husband, Michael Boulos, were cruising the French Riviera aboard one of the world’s largest superyachts — owned by a major broker of Libyan oil.

The yacht, the Phoenix 2, is a floating palace with two helipads, a swimming pool and an 18-foot bronze figurehead of a flaming phoenix rising from its bow. It is not available to charter. But when it last was, it rented for over $1.4 million per week, with the listing highlighting an Art Deco interior and custom Steinway piano.

The vessel is owned by the billionaire oil traders Ercument Bayegan and his wife, Ruya Bayegan. Ms. Bayegan’s energy company, BGN International, stands to benefit from any increase in Libyan oil production.