Thanks for your answers and i am glad you like it
First i thought of making one type of shallow water which fits for all biomes (summer/winter/blackland/desert) to save time (it's really hard to make such a terrain). But i think this wouldn't look good because the colours of the water patterns of each biome are very different. So i will try to make shallow water for each biome (water terrain).
Should this kind of terrain be passable by the widelanders (with roads)? This is an importand question, because if it should be passable, the sandbanks (the grayish parts) have to be on other places than in the above example so that flags are placed on one of this sandbanks.
if they are impassable by boats, not buildable, but passable by roads, how are they different from beach?
Thinking about it, as they are (impassable to both boats and land units) they behave like swamp or other dead land. Except that they can be fished, so ok, they have a difference.
By the way, if trees can be planted on them (or at their fringes) I recommend values of humidity 0.999, fertility 0.1, and temperature equal to the water temperature of that biome.