How do I seal a leak from my overhead tank filled with water without draining the tank?
There is underwater epoxy adhesive available for emergency boat repair. You can use the reinforced PVC material used for inflatable boats as a patch material. The catch is the patch must be applied to the inside of the tank so water pressure holds it in place. Applying to the outside will not work because water will push through an outlet path before the adhesive cures.

How long can you hold your breath? If this is for human drinking water, you should disinfect the water after someone has been swimming in it, on top of sanitizing the person before entry.