My advice is you need to tread carefully here.
A driver that distinguishes between 'clicks' (short press) and 'presses' (longer press) limits your choice of switches when used underwater. A momentary piezo switch works well underwater however a standard piezo can never stay closed long enough for you to impart 'presses'...only 'clicks'. The solution is to use a 'prolonged piezo' which can stay closed for up to 6 seconds, as opposed to under 0.2 seconds for normal piezo's.
I've been making a couple of torches with a similar type driver recently and find for underewater use it is less than perfect to use a Hall switch. You have to carry a second piece of kit, the magnet around to operate it. My experience is the magnet needs to be strong and come at the torch in exactly the right spot to operate. You can do it reasonably well while sitting at your desk but after trying it on 2 dives I can tell you it's a lot harder trying to distinguish between clicks and presses with a magnet, having to use both hands to do it, and while being pushed around by the current. In the end you feel like turning it on and not wanting to play around with it until the end of the dive, which is a waste of the potential to turn it on and off, and use mode-changing whenever you want. I went with a prolonged piezo which is a one-handed job, much more precise (no mistakes) and no fear your compass will be influenced by this magnet hanging off you somewhere. Plus you can change modes simply and quickly without taking your concentration off the surroundings.
If you can work 'clicks' and 'presses' with a Hall underwater then you are a better man than me. What I reckon the Hall is good for are simple drivers, or HIDs. Simple 'click' on/off with one swipe of the magnet, or a torch like a HID which doesn't like to be turned on/off constantly and has no modes. Just turn on at the start of the dive, and off at the end.
A prolonged momentary is the 'Ferrari' of solutions but will set you back $50+, so it's not as cheap as a Hall.
Now... it's fair to say a normal momentary switch does not work in underwater torches because the pressure quicky builds up and forces it constantly on. Some years ago I was too ignorant to realise this and put a momentary on a canister with a thick rubber boot to waterproof it. It worked! and I was blissfully ignorant of the effect of pressure on switches until someone pointed it out. The rubber boot was stiff enough, along with the shape and how pressure acts on the shape to resist pressure so I could press it while underwater and it would spring back. I went no deeper than 20 metres from memory so I am unsure how something like this would work deeper. I might do an experiement to find out. . Point being, this is a cheaper solution than a prolonged piezo, with all the benefts and no disadvantages of the Hall...but...It might not work deeer than 20M and there's the risk of leakage through the rubber boot, which a Hall and prolonged piezo do not have (the piezo has the potential to leak where you cut the hole for it but you only have yourself to blame if that happens).
Pictures of the prolonged piezo on left, and the rubber booted momentary on the right (a starter button from an auto shop designed for boats)
Sorry for the essay but it is not as simple as it first seems.
Good luck deciding.
