How to make complete waterproof e-switch, diving rated? I know piezo or magnet are options but I want try make simple tact switch with more pressure resistance. Any idea is welcome 
You can make a seal using an O-ring. If done right it will be real good waterproof.
To what depth?
that’s the big question, because water weighs a lot.
I did a torch recently where I used a hold-down ring. I wasn’t really going for water resistance (no such thing as waterproof, that’s why there was a ruling in the 70’s against “waterproof” markings on watches), however it essentially sandwiches the bottom flange of the rubber boot against two flats, so it in theory would offer decent resistance.
Pressure is bigger problem for diving. Switch only with rubber cap not have enough force for return back. Need work umder pressure of 10 bar or more
Somehow ,you answered yourself to your question !...The outside pressure will exceed the inside pressure and the e switch will be pressed all the time !!!
That's why , for diving lights are used the Hall switches ( in rotary form or single press )...! In this way , the different pressure out-in will not interfere with the functionality of the switch ( and also will guarantee the perfect waterproof , along with double , and sometimes , triple O rings , at tail cap and bezel )
On the outside you'll have only a magnet , and inside , the driver with Hall Sensors . In time , this system proved to be reliable and , of course , totally waterproof , up to 100 meters ( 10 bar ).
Anyway , I do not think that you can reach this depth , even with special equipment.., decompression could kill , you know...
I know function of hall switch, reed switch, planned stay away from magnets.
I saw gopro housing for deep diving and there is only momentary switches. Maybe something like that?
Adding a spring can counteract the pressure from deep down. It can be calculated how much force you need at what depth and what force the spring needs to be to counteract that.
Obviously there is a point where the spring need to be so strong that you cant activate the switch above water with your hand…
But in the end, waterproofing (or water resistance as DrafterDan pointed out) is all about sealing it up. Using the rubber switch boot can and will work, together with a O-ring you can build a very decent waterproof light.