If its anything like what happened to mine, it'll be the Voltage Regulator which has been damaged. You need to look for the cause though of the water - which is most likely the Rear Wiper's Sprayer - what tends to happen is the tube running to the Sprayer comes off - this is the Tailgate (you'd need to take the plastic trim off to have a look, their are 2 screws (either Torx, or Cross Head) holding on, as well as clips).
To sort out the pump (if you're lucky as I was) you'll need to remove the vacuum tube from the top, then take the unit from the car, then remove the plastic cover on the base (careful prying with a screwdriver).
The circuit board is held in by 1 bolt, you can use a socket to remove this, and the board should come out, with a bit of wiggling.
The voltage regulator which was damaged in my case in one of the corners, and has 6 legs, in my case it had got a bit of gunge on it, and looked like it was a goner, but I convinced my mate to remove it from the board (using a soldering iron, and solder braid), and then we cleaned the chip up. The problem in my case was the gunge was most likely the last bits of one of the legs, so using a leg off an LED, we soldered it in at the base of the leg to the circuit board and top to the remains of the old leg, then soldered all the OK legs back to the board. We then used insulation tape over this area of the board.
We then put the circuit board back inside the Pump, and re-attached the plastic base. We then put layers of insulation around the edges of the plastic.
We reckon the cause of the problem is when water does get into the area around the pump, the fact the plastic has holes in it, allows water to the circuit board. Hopefully putting insulation tape around this area will stop this happening.
In my case I'd already looked at my Rear Wiper Sprayer, and somebody had obviously fixed it before I'd got the car since there was an extra tube around it with a zip tie to make sure it stayed in place. The pump must have just finally decided to give up from the previous damage as there was no water around the pump itself.
I'd try and get the pump sorted since although its true you can open the doors, at some point you'll need to fill it with petrol so you need to open the flap, and the only alternative is to push back the pin by putting your hand in through the boot to where the pin works from, but you'd have to have v slender hands to do it.
Hope any of this helps you. Good luck I spent a lot of a day doing this!!
