Hi. We're thinking of going to Lake Narie near Kretowiny. What's the water temperature like around mid-August?
I went to a lake a few years back around the same vicinity during the month of July, and it was a bit chilly. I also went to the Baltic shores during that visit, and I remember the water being cold, as well.
I think it is impossible to tell now what the water temperature will be half August. Depends on the weather, doesn't it?
On this site you can find reliable information for the actual water temperature for the main Mazurian lakes and the seaside.
Site is owned and maintained by IMiGW, the Polish Meteorological Institute.
We went this past August. Even when the sun was out, the lake was still cold. But it was only during the initial dip. After staying in the water for about 5 minutes, I warmed up a little and was able to tolerate the water. The sun really helped.
I think it's a very nice area if you want a nice relaxing time. However, unless the place you are renting comes with other amenities, then you are basically just stuck with the same routine - swim, boat, or watch the lake, eat, drink, and then sleep. Not a bad deal especially if you want to unwind.
Here is a video collage of my trip to the lake: