Don't immobilize me, dude!

[eltren]

'99 base 986

Backstory: Rainwater flooded the driver's side footwell while the car was parked needing a coolant pump.

Pump, thermo and sundry hoses replaced and good.
Drains cleared and not so good. Water is still coming in somewhere,
slowly. Top is brand freakin new. Ideas?

Most of all, the immobilizer seems to be toast.

The car had been starting oh, half the time after I installed the pump.

Now it won't crank at all. Everything else is fine, as in batt, idiot lights, etc.

So.

How to diagnose definitively?
Can the immobilizer be bypassed allowing the car to operate normally?
If I need to replace the immobilizer, is it a bolt-on job, or...?
If I need an immobilizer, can you recommend a solidly reliable source for a used one?