The BIG problem with water in the trans is that the glue used to apply the fibre to the clutches and band are water soluble - including the torque convertor clutch. GM and Chrysler had problems a few years ago with water entering the trans thru the dip stick tube. Chrysler tried to band-aid the fix before finally replacing everything inside, GM replaced everything the first time. Once water is in there, there's not much you can do but replace the fibres.
If you suspect the radiator or cooler, you can pressure test and /or vacuum test the radiator and the cooler to find the leak. Don't rule out the dipstick tube, or the vent tube/hose as a sorce. D