Saw that nobody responded so thought I’d add. I don’t use a waterproof mic but have. Since I teach outside, at the same places all the time, to an audience of max 25pp, I prefer to teach in and out of the water as do my instructors. (note: in vs. deck is controversial too and most Aquatic guidelines advise teaching on deck).
I find cordless deck mic’s great for very large indoor pools when teaching a big class, provided the facility also has a portable (or fixed) stereo system that allows music to be played through at the same time. Usually these include mp3 plug ins, CD players, and receivers on a cart.
Hope that helps!