unless the water damage includes rotted wood, just leave the area open for a week or so with a fan blowing on the area to dry out the wood. use bleach or TSP to kill the mold. you can use a 1/4 inch screwdriver to test the wood for rot. wet wood will only allow the screwdriver to make a surface dent in the wood. If there is rot, the screwdriver will sink into the rotted area to the depth of the rot.if there is rot between the studs, but not under the studs, I would just dry out the area and not try to replace the plate.