Perhaps Peter Sheard or Kirk Krack could give a more comprehensive answer to your questions, Manglio.
Here's my shot at it:
Underwater symptoms (worsening):
(for deep dives the buddy should spot underwater, meet the diver at 10-15m and follow them up within arms length)
-something looks wrong (trust you instincts, especially if you know your buddy well)
-looking at the surface more often than normal
-panicked look "wild eyes"
-tries to ditch weightbelt
-cyanosis (blue lips and skin)
-body-buckling contractions (O2 deficient)
-stops kicking for more than a few seconds
-grabs the depth line (if there is one)
-grabs safety diver for assistance
-apparent disorientation (possible ear trauma)
-samba (shaking or convulsions) (onset of blackout)
-goes limp (blackout)
-blows air (blackout)
(when watching from the surface any of the above would be cause for concern)
Another variation is a blackout when surfacing from a deep dive or static apnea:
-cyanosis
-signs of distress (for static it could be rapid contractions of the diaphram, grabbing the pool lip, anything that indicates the freediver is approaching his or her limit)
-samba (blackout onset)
-blows his or her air in a stuttering exhale (convulsions)
-goes limp or blacks out with continuing convulsions
Also the hypocapnia (low CO2 blackout):
-the diver may do a tuck dive and come back up within seconds unconscious
-diver over-ventilated and/or had low blood pressure for any number of reasons (dehydration, packing, etc...)
***(Also something to watch for is pulmonary oedema--spitting up blood, coughing fits, pain in chest, wheezing, feeling of fatigue and sluggishness. The diver should stop diving and rest for several weeks and see a doctor. This condition, aka lung squeeze, can lead to secondary drowning as the aveoli are coated in plasma and are not working properly. It happens most often with a big jump in depth, but can happen because of other variables, too. I've had it myself.).***
After the blackout, assuming a recovery in 10 seconds or so, symptoms might be:
-body weakness
-disorientation (there's no way I blacked out! or "what happened")
-sense of lost time
-emotional trauma of many kinds (anger, self-doubt, fear, shock)
-fatigue
-"bad" feeling
-fear of going deep again
-.....??????? There are many other physical, psychological and emotional effects, I'm sure.
***Erik Y. is right about seeking emergency medical help if the blackout lasts for longer than 15 seconds or is very deep (activate EMS, AR, and CPR the next step, while getting the person to shore or a solid platform for possible CPR).***
After a blackout:
-as a buddy you should watch them closely for the rest of the day, even on shore. Fortunately, a black out is usually not unpleasant for the person who experiences it, it's most often the implications that get them. Watch your buddy for signs of shock and emotional disturbance. Their brain has been starved of oxygen, thus possibly doing damage. The body has also undergone acute hypoxia, which is bad for you. (I don't know the specific effects, I'll let Peter Sheard answer that one). Everyone's physiology is different, and complications might result.
-The freediver is DONE FOR THE DAY!!!!!! See above.
-Some say that one day of rest is sufficient, others have felt the effects of a blackout for a week. When in doubt, go see a doctor and get a professional opinion.
Hope that helps (if I've missed anything, I hope the experts will let us know)
Pete
Vancouver, BC
www.holdyourbreath.ca