Very nice to hear from you and thanks a lot for you kind comments on my photo.
No, this is not a reservoir. This is how a natural coast line looks in Croatia. The entire shore is sheltered by a long stretch of islands and therefore right near the actual coast, the water is very calm and has almost no waves. This is why the erosion is much smaller than in other places and there is no sand for example :)
Hope this settles the matter :)
Cheers and take care my friend
Natural blue (34)