If Earth were square instead of flat, we’d all be tipping over and still be stuck to the side. It would just be a very hard 90% turn from one side of the water to the other. On the other hand, Discworld could also be a very thin rectangle with very secure ideas of gravity, which means things fall down. They don’t fall sideways or up, which is what happens on Earth if you think about it properly. Which would mean that the water is just falling over the sides with an evil sort of blue uniformity that is just creepy. However, since the water isn’t falling in any discernible waterfall-y way, but instead, seems to be sticking to the square shape, against all proper ides of gravity, this is most probably a square version of Earth, which is interesting in itself. But still, it’s no square Discworld.