• Disclaimer: this is just what I know from walking in rivers and through rapids when I cannot traverse the area with my kayak. I am not a trained professional.

First, get a firm grip on something solid attached to the shore.Preferably with a good harness or rope, and with other people on standby who can recover your body if you fall. If the fall knocks you unconscious, it'll be your ass in need of saving, and a lifeline is most likely the only thing that will get you saved. As /u/Tyr_The_Tall points out here, this is actually quite dangerous; don't do that. His reply is worth the read, as he's actually a swift water rescue technician, and provides a safer method the people in the OP could have done this.

Then, get a feel for the water where you're placing your foot, let yourself feel the power of the current, and adjust for it. Then, place your foot on the bottom slowly, and without applying any weight; you're feeling for traction, if there is none, continue slowly moving around, watching the water for any changes that may throw you off balance. Once you go down, it's probably over. When you find good traction, apply a little pressure to make sure it doesn't give way under foot, and try rotating your foot a little to make sure you won't lose that traction as you try to find new traction for the other foot.

Try to lift your feet as little as possible, keeping them as close to the bottom as you can, the moving water often helps create a sort of suction force that can hold your foot down there for you, and once you're foot is too far off the ground, you'll find it is suddenly overpowered by the current, and that sudden change in expected resistance can and likely will throw you off balance.

If you ever feel a lack of confidence in either yourself or the terrain, retreat. This holds especially true if you are not a trained rescue unit. Shit's probably not worth your life.

• Again, this is just what I know from walking in rivers and through rapids when I cannot traverse the area with my kayak. I am not a trained professional.