@Weaver...
Obviously, if the rainwater drain channels are blocked then that needs fixed, to avoid corrosive damage. That’s not just bodywork damage, but also corrosive damage to any electronic bits that might be be wetted with mucky water.
But I would be amazed if water accumulation, even mucky water, could be responsible for any measurable battery drain. Water rightly gets a bad reputation for electrics because, at mains potential, it can conduct lethal currents and that much is true. Ohm’s law prevails, for a fixed resistance, the greater the voltage the greater the current.
At 12V, my gut feeling would be, water induced leakage currents would be insignificant to the energy stored in a car battery.