Bummer...
The black piece covering the rear of the hatch just lifts up from the floor. I'd take that out and see what you can see. You might just be able to soak that up with a sponge.
The interior electronics components (i.e., not those mounted outside or underneath) don't like to be soaked.
The only time I've had issues with things opening accidentally, have been when the buttons on the fob have been pressed because of the way it ends up in my pocket sometimes, especially if the pants are a little tighter than my normal stuff.
If it's underneath the rear seat, lots of electronics modules there. The seat is just held on with spring clips and the harder part is threading the seat belts in/out of their slots, but lift the front part of the rear seat up, and you can look in there and take the seat off if required. Putting it back, same idea, get the seat belts where they need to be, then with the back part in place, snap down the front of it to lock it in place.