I just picked up a PC .40 Shield and am looking for a holster that is Level II for retention. I fish a lot and want to carry it under wader etc so it MUST have a retention device be it a button or thumb break. What can you recommend?

Take a look at the Safariland 571 GLS or Safariland 7371

The main difference between the two is the locking mechanisms.

The 571 GLS uses a release that your middle finger of your drawing hand depresses as you draw the pistol, you don't even know you've depressed it.

The 7371 ALS utilizes a thumb release to unlock the pistol during your draw stroke.

Both come with a micro paddle belt attachment, but you can buy a belt slide type attachment.

I will second the Safariland with thumb release. Holds securely.
If you have a thumb safety,your thumb is in perfect location to off safety.