The most common sense approach to emergency response is to remember to refrain from locking one's thinking inside a box ( sorry, that metaphor is grossly over-used! ), by that I mean one must be able to adapt procedures to varying situations. Equally important is to remember your priorities:
> YOUR ( Rescuer ) safety comes absolutely FIRST.
> People live & die by way of A B C & bleeding.
With those primary criteria in mind, react as safely & as quickly as you can. With open circuit equipment, an "unconcious" casualty is likely a " v.s.a." ( vital signs absent ) casualty ( an exception could be a witnessed O2 tox. victim, where the buddy takes control instantly & protects the diver's airway while surfacing...), who has likely drowned. Such a casualty needs the surface & a flat, hard platform ( for resucitation ) as quickly as can be safely managed.
How you achieve your primary objectives can be analogised to a mechanic with a box full of tools, where you are the "Rescue Mechanic", & your "tool box" is your mind's store of retrievable procedural options, adapted to the circumstances you find yourself in.
As to your scenario of an "unconcious" casualty @ 100', consider these thoughts:
> nature of casualty ( who is it? your buddy? An unknown diver you "happened upon?" )
If it is your buddy, you should have a good idea of the circumstances of their unconciousness, provided you were " buddy diving" - not "kinda diving together". You would then base your response on the known circumstances. Much of what you do will be based on your knowledge of your own personal capabilities & limitations. You MUST be honest with yourself, or risk personal injury in trying to do things beyond your limits. This may include allowing a casualty an unescorted trip to the surface because a controlled ascent together was beyond your abilities. Many a well-intentioned rescuer has suffered the consequences of striving to assist beyond what they can safely achieve, often with tragic results. Resist.
If you "happen upon" an unresponsive diver, they have likely drowned. You now must first consider your own circumstances:
> what is my buddy & mine's dive schedule? ( that's right - you SHOULD have another person along to assist in recovering this person to surface! ).
> How do "we" safely, effectively, get this person to the surface?
You priorities are now:
> your safety
> their airway ( protect it as best you can )
> their buoyancy ( control it as best you can )
> the surface ( as quickly & safely as possible )
Once on surface:
> all get positive
> check breathing. No breathing? Try several rescue breaths as best you can, if there is no response to ventillations, then...
...decide - how close is an adequate platform for c.p.r.? If your close, make all speed for it ( I personally wouldn't slow our speed up by trying to breathe for the casualty en route - they need circulating blood to move any O2 I give them, & I can't move their blood in the water ).
...if said platform is a long way off, tow & breathe for them the best you can, & hope for a miracle.
...then again, hope for a miracle always.
Best Fishes,
D.S.D.