Must one repeat the ghusl if urine is discharged from the person performing ghusl while their entire body is submerged in bathtub water, thereby potentially contaminating the water?

If the water you are bathing in is little (less than two qullas), it becomes impure upon the entry of urine into it. You are then required to purify any part of your body that came into contact with this impure water.

If you immersed your entire body in the water before urinating, your ritual impurity (hadath) has been lifted, and you are not required to repeat the ghusl to lift the hadath. However, you must wash any part of your body that became impure.

But if any part of your limbs remained unwashed before urination, you must wash it with pure water with the intention of lifting the hadath.

It is forbidden to intentionally urinate in the small amount of water used for ghusl, as it leads to becoming soiled with impurity without necessity.