About a half hour before reaching Helsinki harbour on a ferry from Tallinn, Estonia, passenger Mikko Silvennoinen was taken aback by the sight of jet skiers leaping over waves behind the much larger vessel.
However, the practice is fairly common during summer, according to Baltic ferry operators, particularly in waters near Stockholm, Sweden.
Stefan Ölander, who captains the Silja Serenade ferry on voyages between Stockholm and Helsinki, said he has occasionally seen similar ski jet antics in the Swedish archipelago for years.
However, the jumps over the wakes of ferries can be dangerous, according to Ölander, and can prompt collisions between jet ski riders.
Similar observations, mostly in June and July, have been made on Viking Line's XPRS ferry which shuttles between Tallinn and Helsinki.
The Finnish Coast Guard's Deputy Commander Mikko Hirvi said the agency receives reports of jet skiers jumping on the wakes of ferries on a weekly basis during hot summer weather.
He noted that such activity poses high risks to jet ski drivers and possibly others.
"We always assess such matters on a case-by-case basis in terms of regulations, but it could be a question of skiers causing a hazard, for example. It's also clear that those kinds of tricks can disrupt passage of ships and always pose a danger to jet ski operators themselves," Hirvi said.