Have you considered a diving light like an Ultrafire DV-S9? 26650 cell and lots of runtime, meant for diving, so it’s definitely gonna be waterproof.
Magnetic slider switch (slot, small plastic piece with magnet inside), and continuously adjustable from off to 100%. You can “swat” the slider even with gloves to whack it on/off if you’re not terribly interested in intermediate brightness levels.
The whole light is essentially sealed, 3 O-rings on the tailcap, and the front is probably glued shut (never tried opening it). No discrete switch for water/dust ingress at all.
You can use spacer rings/sleeve to use an 18650 instead of a 26650 if weight would be an issue.
See Ultrafire DV-S9 diving light? for my first impressions of the light.