If you are in fresh water, those might work ok for a while. We had about 6 of those Mach1s and they didn't last long at all. We have Brackish/salty water here. The magnets used to power them up started to corrode, stopped functioning completely after a while.
I had a jump lesson with one when I first started kiting and it was good. Audio clarity was superb. I don't think the manager of the school rinsed them down with fresh water or anything so that probably contributed a lot to how they deteriorated in a short time. Plus they were an issue to keep a charge. But like anything take it with a grain of salt.
There's an inexpensive way to rig helmets you might already have. Buy some cheap radios that are small, put them in an unlubricated condom, put that in a dollar store cell phone case and clip that to the helmet, making sure to velcro the closure of the case and tie the clip securely to the helmet.
Gotta use a fresh rubber each time but it works fine, just have to fiddle with the volume to keep the student from going deaf. One advantage is if you keep extra batteries around and rubbers on your watercraft, you can fix the problem if you forget to charge your double AAs. If you forget to charge your $400 radio helmet, you are out of luck.
It works pretty well, I just wouldn't draw attention to the rubber wrapped antenna coming out of the cell phone case.