The 16-hour trip between the two northern European cities is a festive summer ritual, with plenty of singing, gambling, limbo contests and maybe a bit too much to drink.

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

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