I had water entry into my red Box only a few days after I bought it. Porsche engineering decisions sometimes mystify me. Water-sensitive electronics located in the lowest interior point in a convertible, with no water drain??
Thom 4782 is right; the guys at Specialized ECU Repair, in Oakland Park, Florida,
Specialized ECU Repair, Replacement, & Testing - Restore or Exchange your Electronic Control Unit (ECU), know their stuff well. You will have a more sure-fire solution, less total VOR (vehicle off road) time, and less total expense to send your parts to them. They will need your immobilizer box, your engine control module from in the rear trunk, and all the keys.
They also have a waterproof box that replaces the factory one; it is easy to install, requiring only removal of the seat above it. They even include a Torx socket for removing the seat, very thoughtful.
I have fitted that box to all three of our Boxsters, and also--Horrors!!--drilled holes in the floors to allow water to drain BEFORE it gets deep enough to threaten the immobilizer. I suggest every new owner of a Boxster not already fitted should fit this box upon acquiring the car.