Create a displacement map from the surface of the lake- take the saturation down and crush the black and white with levels (not too much). Use that to apply a distortion to your 3D creature as it moves under the surface. Duplicate the lake layer and comp the creature in between. Play with transparency on the top layer to see what looks best. You may need to apply some color correction to your 3d creature to bring it closer to the color of the water. Try creating an object with the same color/texture as the creature and shoot that under water in the same day/time as your lake shot for reference. You can use that to tweak your final comp.