The party started an hour before the M/S Gabriella ferry slipped out of Stockholm harbor for its overnight voyage to Helsinki. At the outdoor bar on Deck 9, passengers had gathered for the 4 p.m. sailing. Talking excitedly over drinks, they reveled in the Nordic summer weather — bright sun, cool breeze, electric blue sky with cotton-ball clouds — while a D.J. mixed bass-heavy Eurodance and vintage disco. Kids on holiday with their families dashed about.

Ferry trips on the Baltic Sea, including the 16-hour ride between Stockholm and Helsinki, are practically a summertime ritual for many of the region’s vacationers, with nicer cabins booked out far in advance. The ferries are known (infamous, some might say) for their party atmosphere, and though the fun has a more family-friendly vibe during the July to August high season, there’s no shortage of grown-up high jinks, no matter what the time of year.

My husband, Mac, and I were part of the group on Deck 9. We’d gone to the bar after dropping our luggage in our cabin and, soon after, beers in hand, had fallen into conversation with Hanna Bäckman and Anna Dahlman, two Stockholm residents. “The last time we took this trip was 29 years ago,” said Anna, a real estate agent, who explained it had been a secondary school outing. Hanna, a hydropower project manager, laughed. “Yes, and we plan to have as much fun on this one,” she said.

A buzz rippled through the crowd as the 11-deck ferry carrying cargo, cars and 2,000-plus passengers gently pulled away from its berth. For the next several hours, the ship would thread its way through the Stockholm Archipelago, a dense constellation of skerries and forested islands dotted with barn-red wooden cabins and fluttering blue-and-yellow Swedish flags, before crossing open stretches of the northern Baltic Sea.