In the deep water areas, there is no light reflected from the sea floor, so the majority of the incoming sunlight is absorbed by the water. This is where the darkest blue is found. Water molecules absorb reds, greens, oranges, and yellows, but relatively high frequency blue (and violet) is not absorbed to the same extent.
Some sunlight is directly reflected from the sea surface, but the majority passes into the water.
In shallower depths, the color you will see depends on the composition of the sea floor, dark rocky areas will reflect less than lighter sand based seafloor. In the picture above you can sea that the areas near the shallows appear a much lighter blue than further out to the deeper sea.
Variations in sunlight due to clouds can also result in color changes.
Finally, the color can be influenced by phytoplankton, whose chlorophyll absorbs red and blue light and reflects green. Depending on the concentration levels of these tiny marine organisms, the sea will take on different shades of green.
The sharpness of the adjacent colors in an indication of how abruptly the depth increase or decrease, how quickly the color of the seafloor changes, the concentration of phytoplankton and, unfortunately the amount of sea water pollution.