The underwater feeling is somewhat variable. There are completely sealed IEMs that do this, and there are vented IEMs and semi open IEMs that are not nearly as bad.

There are some fully open IEMs as well, but they are definitely less common. Audeze's IEMs would qualify.

Cable microphonic are helped a lot by things like running the wires over your ears and shirt clips. Basically things to absorb the vibration before it gets to the actual ear piece.

Probably not much help with chewing or breathing, but still less noticable with vent d or semi open IEMs.

I use IEMs somewhat regularly and am generally amazed at the sound they can produce. But also don't really enjoy shoving things in my ears, but sometimes it's worth it for portability and/or isolation.