Between May and July, 651 rescue operations were carried out across the country, the highest number for that period since the Swedish Sea Rescue Society (Sjöräddningssällskapet) started its records in 2000.
July was the month when rescuers had to work most intensively, with 350 operations, following a record June in which 214 interventions were carried out.
The summer's unusual heat, attracting more people to Sweden's lakes and coastline, was the main reason for the increase, according to the Båtliv newspaper which published the figures.
It was the third year in a row that the number of rescue operations at sea increased for the May-July period, with the 2018 figure 27.8 percent higher than the average over the past five years.
More than two thirds of the rescues (70 percent) related to leisure boats, while 14.5 percent were linked to people without a vehicle, such as swimmers. The remaining cases were linked to merchant vessels.
Figures from the Swedish Lifeguard Society (Svenska Livräddningssällskapet) released earlier this summer showed that the number of child deaths from drowning in May and June alone was equal to the number for the whole of 2017, and the number of overall drownings was also up.