Up or downwellings can occur were ever there are currents. Normally seen on walls with strong tidal changes or standing currents. As others have said they are normally narrow and can be swum out of by trying to fight it while swimming at an angle away from the wall.
Downwellings:
I would suggest attempting to swim against it and inflating the BC if this does seem to be working angle the swim away from the wall. If your rate of descent continues drop weights- DCS is curable drowning isn't. Once the downwelling is escaped from be prepared to dump the BC and flair out if weights had to be dropped to slow the ascent
Upwellings:
Same procedure in reverse swim down and away, dump any air from BC-- Remember to breath The most dangerous thing in an upwelling would be an embolism from rapid ascent
I have seen many dangerous guides in my years of diving and some very good ones. A diver has to rely on themselves to make decisions on if its safe to dive and to handle problems underwater, a good dive guide will minimize problems and make dive more pleasant unfortunately there are many poorly trained guides that contribute to the problem instead of avoiding it.
Downwellings:
I would suggest attempting to swim against it and inflating the BC if this does seem to be working angle the swim away from the wall. If your rate of descent continues drop weights- DCS is curable drowning isn't. Once the downwelling is escaped from be prepared to dump the BC and flair out if weights had to be dropped to slow the ascent
Upwellings:
Same procedure in reverse swim down and away, dump any air from BC-- Remember to breath The most dangerous thing in an upwelling would be an embolism from rapid ascent
I have seen many dangerous guides in my years of diving and some very good ones. A diver has to rely on themselves to make decisions on if its safe to dive and to handle problems underwater, a good dive guide will minimize problems and make dive more pleasant unfortunately there are many poorly trained guides that contribute to the problem instead of avoiding it.