My guess is maybe not even a cat but a racoon. Cats tend not to do their business in water, but like to bury it - racoons on the other hand will often stop by waters edge to wash a food item or their front paws, and will leave a calling card right there where they washed up! Their droppings look just like cat droppings. We have Koi ponds and when our dog was ill and layed up, the racoons moved in, tried to help themselves and left calling cards right there at the edge of the ponds.