"While the Baltic freezes in winter".
I know that has happened but not that often completely so I had to check it.
"It is known that since 1720, the Baltic Sea has frozen over entirely a total of 20 times. The most recent case was in early 1987, which was the most severe winter in Scandinavia since that date."
"On the long-term average, the Baltic Sea is ice-covered at the annual maximum for about 45% of its surface area. ".
That 45% is in the northern part between Finland, Sweden and Estonia. And not for that long.
But I do agree most summers are fine and the water is warm enough, but it gets a lot colder if you dive a bit.