Did you spend the summer of 2010 here? A new all time heat record, 37 C, was made in July 2010. The highest temperature measured in Tampere then was about 33 C. We are a whopping four degrees shy of the all time record this year. May was exceptionally warm. But June had long periods cool weather. The first week of July was nothing to write home about. Since then it has been quite hot but we've seen hotter.
In late July and early August 2007, the surface temperature of Näsijärvi, your local lake was 27 C. It's nowhere near that high, yet. That's because it's been quite windy for the past few weeks. Lake surface temperatures are not very much above average for the time of year. The usual range is, as far as I recall, about 16-24 C depending on wind conditions and the weather in general. If large lakes such as Näsijärvi warm up to 25 C or higher, we're talking about an unusual situation.
Follow this link if you want to see a map with a few dozen lake surface temperature readings all over the country measured today.
For me, subjectively, and I very much dislike the cold, swimming in waters between 18 C and 23 C causes minor discomfort at first lasting between about fifteen to a few seconds. 15 C or below is far too uncomfortable for me to even think about dipping into. You'd have to be exceptional for 20 C to feel frosty to you.
If I remember correctly, I was in Italy during those days. However, it has been reading in the low 30s around here at pretty much everyone's house. they are not so accurate though.
Yeah, outdoor thermometers can be off by several degrees compared to the ambient temperature weather stations measure. Temperature is measured from inside a wooden box painted white protecting the thermometer from direct sunlight and with good air circulation, usually in the middle of a lawn at a height of two meters from the ground. Those readings tend to be lower than outdoor thermometers typically attached to a window frame.