Hmmm, maybe I was just unlucky and received a defective part then.
I can now say with certainty that the issue was in fact the idle air valve. I installed the new OEM Mercruiser part last night, and it completely resolved the idle issue. Boat runs great now!
As a note for anyone else that goes to change their air idle valve, removing the air intake plenum (black cover with the scorpion logo on it) requires you to remove 2 nuts that are hidden beneath towards the rear of the engine. I had figured just the 6 torx bolts on top would do it, and it took me a while to find the other two nuts. They are a pain to get at!