The ARC-AAA can be operated one-handed as others have said. I do so very easily with mine. The amount of twist you can apply naturally between thumb and forefinger, about 1/4 turn, is sufficient to keep the light safely in the OFF position, and easily turn it back ON without contorting your hand.
If the action on yours it too tight, the O-ring can be tweaked to make it easier to turn. Slip the O-ring onto a AAA battery and rub it against some very fine grit sandpaper to reduce its OD until it's to your liking, then lube it. That's what I did.
The AAA is very waterproof. The electronics and LED are sealed with epoxy so even if water gets in the tube (unlikely) there is no harm done.
I sometimes use the momentary-on that tardis mentioned, by twisting the head almost ON, then pushing sideways with my thumb on the head. Works well. Just be sure to turn it 1/4 past OFF when your done to avoid it turning on accidentally.