How can I convert the blue color of water into another even if there are different shades of blue present?

Question

How can I convert the blue color of of the water in an image into white, especially if there are different shades of blue. And then separate any other color and give it for instance black. An example would be this pollution in the sea and you want to detect the contaminated area and give it a color clearly distinguishable from the water:

Answer 1

import cv2
import numpy as np
from matplotlib import pyplot as plt
BLUE = [255,0,0]
img1 = cv2.imread('11.jpg')
replicate = cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_REPLICATE)     
reflect = cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_REFLECT)
reflect101 = cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_REFLECT_101)
wrap = cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_WRAP)
constant= 
cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_CONSTANT,value=BLUE)
plt.subplot(231),plt.imshow(img1,'gray'),plt.title('ORIGINAL')
plt.subplot(232),plt.imshow(replicate,'gray'),plt.title('REPLICATE')
plt.subplot(233),plt.imshow(reflect,'gray'),plt.title('REFLECT')
plt.subplot(234),plt.imshow(reflect101,'gray'),plt.title('REFLECT_101')
plt.subplot(235),plt.imshow(wrap,'gray'),plt.title('WRAP')
plt.subplot(236),plt.imshow(constant,'gray'),plt.title('CONSTANT')
plt.show()
cv2.waitKey()
cv2.destroyAllWindows()

Answer 2

import cv2
import numpy as np

from PIL import Image

import colorsys

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.image as mpimg

def rgb2gray(rgb):
return np.dot(rgb[...,:3], [0.299, 0.587, 0.114])

img = mpimg.imread('0.jpg')     
gray = rgb2gray(img)    
plt.imshow(gray, cmap = plt.get_cmap('gray'))
plt.show()

cv2.waitKey()
cv2.destroyAllWindows()