Thirty-seven-year-old Mary, who enjoys golf and horse riding among other sports, was pictured in Porto Cervo with her husband Crown Prince Frederik and their children Prince Christian, three, and two-year-old Princess Isabella.
The family group, who were in town for a sailing competition, took to the Mediterranean in a dinghy - and Mary pulled on a pair of goggles to go for a dip in the inviting waters.
Having grown up in costal town Hobart, Tasmania and after living for some time in Melbourne - which is also by the ocean - Mary is more than used to the seaside.
And the glamorous royal was clearly thrilled at the chance to have some aquatic fun with her family in the sunshine on their holiday.
