A pathfinding solution

Processing
7

Almost,

d (distance) = (herbivor.x - water.x)² + (herbivor.y - water.y)²
That is simply the pythagorean theorem but without the square-root :

To compute the smallest one you need a variable to store the current smallest. In macro code in would be something like this :

int currentSmallest = height * height + width * width; // You initialize that variable with the biggest distance you can found AKA the diagonal length of your canvas

for each waterPixel in waterEdgePixels { //You go through all your water edge pixels
  d = (creature.x - waterPixel.x)² + (creature.y - waterPixel.y)²; //You compute the distance to your creature
  if (d < currentSmallest) { // If the distance is smaller than the smallest that you found before then you found a new smallest
    currentSmallest = d;
  }
}