I thought you would have already pulled the foam out to check flow. You can check the duty cycle of the IAC during start and warm-up with a good scan tool. After a few minutes warm-up the duty cycle will be significantly lower (20-30%). The IAC will usually throw a code hard or soft, depending on the condition of the system. However, if the IAC muffler is clogged or melted, the IAC may not read correctly. You can run without the foam if you find it clogged or damaged, it will just be noisier.
Fuel injectors? Really? I'm surprised to hear that suggestion.
Yes, really. It is frequent to have idle issues and not have them at higher RPM, just like the IAC. This is no different that you might see in an auto. Boats are usually low use engines and fuel fouling can happen (as opposed to worn out injectors). No one knows how you run your boat better than you. The same goes for regular maintenance. If you are running a lot of fuel through it, then this may not be an issue for you. For others, fuel stabilizer can be your best friend. I hope it is the simple 5 minute fix.