I'd change to an independent external cooler. Then, I'd have it towed to a shop that has a BG flush machine. What it does is, you fill the machine with 12 qts of new tranny fluid, then as it runs, the new fluid goes in while the crap fluid comes out. ( Kinda a fluid transfusion ) Then see how it goes. Water does bad things to a tranny, and even if you do all that, you could still be looking at major tranny damage in the future from having this have happened.