Well, my problem is all about tension. So I have made 2 pieces of crochet to find the tension of this project. First I made a simple granny square, counting 28 stitches at the sides, and compared it with my first piece of the blanket, it turned out to be a bit smaller, with an extra row of sc on top, the size was better. The sides are now 15/16 cm and count 28 stitches.
I also made a square of rows of 28 sc stitches. It turned out to have the same size of the blue square. I am now convinced I can continue with the original patterns for only the water pieces will be a bit too large. The height will be the same as the squares to come. My blanket will in the end be a bit larger.