As mine is parked 'up hill' so water doesn't drain out the bottom drain holes therfore I have drilled a small hole in the corner of the door just behind the rubber door seal which enables me to drain the water out when the door is opened. nb as the drain hole is between the seal and door skin it stops water coming out around the seat belt area).
I would always try to park down hill. I use decorators PVC dust sheet plastic from B&Q. £6 buys enough for at least 2 doors and a load spare. Use 3M double sided sellotape type stuff to hold it on. Tucked in at the bottom as it should be. The other thing a water puddle by the seat belt mount is doing is rotting the cab floor out round the seatbelt bracket. I would really always park nose downhill.. This looked a bit like damp carpet. One good pull up of about 20 lbs.. Most of that came from underneath.