the TCM monitors the thermistor in the gov pressure sensor to determine trans temp. this resistance value is around 1000 ohms (to ground) when cold. as the transmission heats up, this resistance value decreases.
water is a conductor. when the wiring gets wet, it lowers the resistance to ground on the trans temp circuit, fooling it into an overtemp condition.
my first step would be to follow the wire harness from connector on the top of the transmission and check for damage. if nothing obvious, disconnect harness from transmission, clean connectors, use dielectric grease, reassemble.
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