The Gates family was spotted enjoying the sun on the The Serene, which is owned by Stolichnaya vodka magnate Yuri Scheffler.
The yacht, currently moored off the coast of Sardinia, features 12 state rooms, a seawater swimming pool, a movie theater, a nightclub, a climbing wall, a gym and two hot tubs. There's also a full library, health spa and underwater viewing room.
A helicopter can take guests to and from solid land.
Gates, whose wife Melinda and children Rory, Jennifer and Phoebe are reportedly aboard the yacht, is worth an estimated $76 billion.