I can't say that I'm an expert with down currents, but I would say I'm fairly experienced. I encounter 6- 10 per year. Most are annoyances, perhaps once every 2 years a big one gets the heart rate going.
Unfortunately most articles on down currents and the way to survive, are nonsense. Clearly written by persons who've never been in one. (Perhaps I should put together some text for someone to turn into a polished article?)
In the case you highlight, with a down current on a reef, you're absolutely correct, in that it's wise to climb out. For that reason I always carry a SS muck stick on my wrist (aluminum ones aren't potentially strong enough). At the first sign, poke it into a hole, stabilise yourself, and then inflate and climb out (diagonally)
People say to swim away from the reef. I say, good luck with that. You don't know how bigger area the down current covers and if you don't have a hard bottom you can be going down a long way
I once got blown down from 25m to 50m ish before I could stop myself. The sand was another 50m below me.
If you are in blue water, then your options are limited to inflating and fining hard horizontally (pick any direction) Diving in such locations requires a decent set of fins to enable you to kick hard, Also you need the fitness to do so for 100m.
I always have my computer alarms on, the ascent warning can be the only indication you're going up as it's easy to become disoriented or get tunnel vision.
Being aware of the area and importantly altering your gas plan for added contingency on that dive. I generally dive 15l steels filled to 230bar. I end the dives generally at 90bar and my consumption is around 13l/min.
When it's all going a bit crazy, I've surfaced with only 20bar after less less than 45 mins underwater, and my consumption has more than doubled. My worst case was that my 15l and my Al 40 pony were almost sucked dry.
An unfit diver working hard in current and being "excited/panic" is going to suck through gas at an alarming rate - I've seen that before had had to donate gas.
Leaving depth with min contents in an Al 80 and then getting smacked by a current is a recipe for disaster.
I can't say there are many tell tales. Fish sheltering or working hard trying to swim upwards is a good one. Fast changes in water temp not so much. With a sudden hit of cold water more often than not there is no down current, but occasionally there is.
My personal favourite (not) is watching my dsmb launch and head to the surface at a 45 degree angle because of current (quite normal) then it stops going up and starts to come down (bad, very bad) this is normally accompanied with lots of profanities through my reg as I prepare for a ride.
Hopefully you won't encounter a proper down current - always a risk in Indo though
