Water doesn't come in slices. If you have a glass that's half full of water, and then go on to fill it with more water, the result is a glass full of water. There's no way of knowing which part of the water was in there to begin with, and which part has just been added. It all blends together. Isn't that how water works?
Actually, not always. What you see in this picture is two forms of water that are just too different to merge. The lighter blue water is fresh water, running off a melting glacier, whereas the darker blue sea water of the Gulf of Alaska is denser and saltier, and therefore can't mix together with it. As a result, the sea stands at an impasse, marked by a frothy white line where the two types of water clash and rub together.