the following wording might be better:
A sea serpent, 45 metres long, lies hidden under the surface of the water. The animal is bent horizontally and vertically, but doesn't touch itself anywhere, not even diagonally. Its head (1) and tail (45) are visible, as well as one other part of its body (23). The figures outside the diagram indicate how many parts of the snake can be found in each row or column. The black squares are rocks where the animal can't be. Can you establish its exact position?