I don't think nonlethal damage can kill you, though. We'd have to specify that it starts taking lethal damage once conscious or something.
I don't think you have to refer to drowning since suffocation has its own section, but that's a pretty good solution. Sure.
Here's a first go at it:
Crystal Storm (Su): Once per day, a demarax can unleash a 20 foot burst of whirling crystals. Any creature within the burst must make a DC X Reflex save or be struck by a crystal shard. A struck creature is afflicted by one of the effects at random from the table below for 2d4 rounds:
1 - dazzled (–1 penalty on attack rolls, Search checks, and Spot checks)
2 - fascinated (takes no actions other than to pay attention to the fascinating effect)
3 - color spray (as the color spray spell)
4 - sickened
5 - confused (may act, may not, may flee, may attack nearby creatures)
6 - baleful displacement (50% miss chance when attacking opponents)
A demarax uses this attack only as a last resort, since it uses up the creature's supply of nutrition. The demarax takes 2d10 nonlethal damage per hour until it feeds on a crystal. Upon falling unconscious from this malnutrition, a demarax begins to suffocate.
That's a fine change. Should we say it can't heal the nonlethal damage until fed, either?
I mean through natural healing. Normally, you heal 1 hp nonlethal/HD/hour. It's probably not as much as it's taking, but it's not entirely insignificant, either.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.