I mostly don't agree with this.
Twisties like the C-LE, the L0DCE, or even the Arc AAA due to their "one piece" body construction have less points of entry making them more water tight. BUT, they are also more prone to leak than a properly-maintained clickie when operated under water.
If you're turning it on in air, then taking it underwater and not twisting again until it's back in air, it should be fine. But if you twist it under water, it's much more likely to allow water in.
A properly-maintained clicky can be operated (as in turned on and off) under water much more reliably than a twisty can. By "properly-maintained", I mean lubed o-rings (which applies to both) tight tailswitch boot, etc.
If your tailswitch is installed loosely and not pressing on the boot properly, it will leak... which I suspect was the problem from previous reports. Fenix tailswitches are somewhat notorious for loosening up.
Sorry, no offense, but your impressions are simply wrong in this area.
I am a scuba diver with hundreds of dives logged, and have owned several dive lights, as well as a high end Nikonos underwater camera with underwater flash and underwater light meter housings. All this equipment is rated to operate at maximum depths ranging from 30 meters to well over 100 meters, and
every single one of them uses o-ring seals for ALL control inputs.
NONE use 'rubber boots' like the one on the tail switch of your flashlight for anything .
There is a reason for this. An o-ring is designed such that when it is under high pressure it will wedge it's way to an even tighter seal rather than leaking. Even if it is disturbed by rotation as you turn on a twisty light, it normally won't leak a bit if properly lubricated, because the pressure on it is holding it in a tightly sealed condition.
On the other hand, the flat rubber edges of the typical tail switch boot, are pretty much the WORST way to try to maintain a seal at high pressure, because as water pressure pushes on the surface, it distorts in a way that lets water flow instead of cutting it off like an o-ring. Flexing the thing around by mashing on the button while underwater would just make this worse.
The best 'boot' design will have a thickened perfectly circular rounded edge (like an o-ring has been molded into the edge), so the edge of the boot can be trapped in a seal gap and work like an o-ring. By using this integral edge o-ring seal, and filling the void under the switch with non-conducting and non-compressible silicone oil, it is possible to make a membrane switch that will maintain a seal and work properly at more than 100 meters depth, but unfortunately I have NEVER seen such a sophisticated boot design on a flashlight. Even with that type of edge seal, the tail boot on a flashlight is subject to a lot of wear, and the rubber can develop porosity and tiny cracks over time, and these defects can be so small as to be completely unnoticeable UNTIL YOUR LIGHT LEAKS.
