The only time you can engage the lock is when it's in the OFF position.
I find with my light, the switch is quite distinct - you can definitely feel the click once to the ON position, and then a second click for the High Beam position.
I really do like this light. Looked at the rechargeable - too rich for my blood. I always use Duracell batteries in my light - a little heavy to carry, yes. But they seem to last for a while. I have never counted the number of dives, but my guess is, I did not have to change the batteries for 4-5 night dives (and I usually leave it on the high beam).
I have seen people using HID - that's really way too bright for me. If your partner or anyone you're diving with is not using HID, you tend to ruin their "night vision" when they are using regular halogen light (ie, they can't see as well when your light is not shining at the objects they are looking at - it's like an on-coming car having its high beam on). The HID is nice for taking video at night, but that's about it (IMHO).