Perspective error when scaling on a picture

For a project I need to use the scaling tool to measure distances of a larger object under water. I would put a camera in the deepest part of a swimming pool that takes pictures upwards. Afterwards I would use the scaling tool on imagej but I’m wandering what the error is or how to measure/rectify the error you get by taking a picture from a distance (3-4m). I would believe that setting a scale in the middle of the picture gives of an incorrect result when measuring something on the far right or left side of the picture. Is that correct?