@Diving Dubai had an interesting incident.
Basically, if you end up with a strong down current you may end up a lot deeper than planned for a lot longer. In his case he had an AL80 as a pony and that was barely enough.
Nobody ever complained that they had too much gas with them.
Since I was linked... Just to correct Rainpilot, on the dive in question I had a AL40 as a pony. Even so, prior to that incident on that site after a 1 hrs dive to a max of 30m ish, I would expect to surface with 80-90bar (45cuf) on my backgas and obviously the pony untouched. The dive in question I was down to 15-20bar in each cylinder (around 12cuf in total)
Long story short, on a club dive we went from a position at 30m of being well within out Rock bottom gas and NDL to being at 56m, and having to climb out against the current. At 6.6 ATA with elevated gas consumption (understatement) you can see your gas contents diminish before your eyes.
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@Wingy I love currents - but if you dance with the devil you need to be prepared to get spanked sometimes.
My tips.
Listen to the dive brief if you're on a trip. The guides and staff have dived the site a lot and so have a good understanding.
Follow the guide and watch carefully - they'll often have the first indication.
If you're on a site with known currents, watch your gas and NDL. Don't push the limits. If you get into a position where you need so contingency and it's not there, well you'll be disappointed at the very least.
If you have little experience with currents, don't go jumping into a maelstrom expecting it to end well. Start on easy sites and build experience.
On sites, keep low and use the geography to protect you - don't fight current - it will always win. Watch your gas. If it all goes pear-shaped you can easily double your normal gas consumption
Reef hooks are good, for sure. They're useless however rolled up and stowed in a pocket. On sites with a known current mine is always clipped off to the harness, the line is shortened (to prevent entanglement) using a daisy chain knot with the hook clipped to a D ring using a double ender. If I need it just un-clip the hook and the line extends.
Reef hooks however are good to hold yourself in a current. I prefer my metal pointer stick which is always in my hand and you can jab it into a rock for immediate assistance.
As
@northernone stated, disorientation can happen, head for the brightness. I've personally never had a mask come off. Partially flood, had bubbles stay in front of my face and have been tossed around like a rag doll for sure, but in extremes and not where a commercial trip is likely to take people.
If your on a wall and have stopped, take a moment to think assess the situation and control your breathing. Hopefully you'll see everyone else and can follow the lead of the guide
Definitely have a 6' dSMB with a decent amount of line on the spool. I have 45m (147') of line on my spool. Just launching from 20m (60') I've almost emptied the spool because of a mid water current above me. Know how to deploy a dSMB and not just from the 15' stop.
I've heard all sorts of theory's about using a dSMB or lift bag for additional buoyancy - in a current good luck with that, however if you're in teh blue and going down you have nothing to lose.
If you have arrested your descent, pay attention to your computer (disorientation) if it starts screaming at you, it'll be an ascent alarm. Don't have an uncontrolled ascent.
Big down currents are very rare because guided trips tend to avoid those sites when the conditions get iffy, they are easily survivable, just think things through before hand and
always be conservative
