Discerning mild current.

[tursiops]

@Kharon
For reference, 0.5 m/s is about 1 knot. So, just hover or float for 10s and estimate how far you have drifted by looking at a stationary object....if you've drifted (say) 10 feet, then the current is about 3m/5m = 0.6 knots. A typical recreational swimming speed is about 100ft/min or 30m/min, or 0.5m/s, or 1 knot. That means if your test drifted you 10ft=3m in 10s, swimming with the current will take you twice as far as you'd go without a current, and swimming against the current will take you twice as long to go the same distance you'd go without a current.

For additional reference, Navy SEALS abort a mission at 1.5 knots.

P.S.
For me, drifting 5ft in 10s is a mild current, 10ft is modest, and 15+ is strong.

 

[scrane]

Which direction are the fish that are trying to maintain position pointing.

 

[nolatom]

Current isn't hard to discern if you have a fixed point to compare it to, typically the ground beneath you. And I think we tend to overestimate both our speed while finning, and the current speed. It doesn't take much current to make you wish you weren't there in the first place, and really regretful if you proceeded "with" the current to begin your dive, rather than against it.

It seems the average diver finning with average effort and no need for urgency, makes about half a knot (50 feet per minute), as discussed in this thread:
Just how fast is the average diver
I have found diving in half a knot a lot of work when going up-current, you breathe hard, wear out your legs, and use air too fast.

And near-maximum effort finning, I understand to be about one knot (100 feet per min). I (very "average" diver I think) personally have no business diving in a one-knot current to begin with, nor does a boat or DM want to put you into that challenge if the boat is going to remain at anchor.

A one-knot current does, however, make a wonderful drift-dive, where the boat drifts above you, and there's no "going back" to an anchor line. That's what we do off the SE Florida coast, with the fringes of the Gulf Stream moving us along. You glide quickly over the reef with finning minimally and mostly just to "steer" rather than propel you. You cover lots of ground and your air lasts a long time. It's "heaven". Going in the other direction would be, uh, that other place...

 

[scubadada]

I do quite a bit of drift diving in Boynton Beach, Jupiter, and West Palm. It is usually quite clear which direction the current is flowing. It generally flows north, but sometimes it is south, sometimes there is virtually none, sometimes it even changes directions during the dive. If it is not clear which direction the current is moving, I will let go of my flag reel at the depth I will be swimming, and watch it for a while to see which way it goes. I'm sure this is not perfect but it does tell me the direction my flag is moving and will pull. I often dive by myself and generally just want to go in the same direction as other divers and the boat. When there appears to be no current, the convention is generally to go north.

 

[johndiver999]

Also estimating current speed is an important skill
and you can very easily be deceived. If you are drifting with a mild current and kicking along ever so gently, you just feel like a good strong diver. But turn around and swim against it? Now the current feels twice as strong.

If you are diving in a situation where you are going to have to move against a current or one develops while down, spin around and see how it feels to go upstream before you get too far from home. It is so easy to be deceived and end up down current and then being faced with a much harder swim back than you expect. You get tired and air use can double or more on the way upstream.

 

[tursiops]

If you are diving in a situation where you are going to have to move against a current or one develops while down, spin around and see how it feels to go upstream before you get too far from home. It is so easy to be deceived and end up down current and then being faced with a much harder swim back than you expect. You get tired and air use can double or more on the way upstream.

Excellent advice!

 

[60plus]

I watch for bits of weed or particles moving in the water and their speed relative to the anchored boat of bottom. Came across an interesting phenomenon recently, the buoy indicated the current was flowing to north east and this was the case at the surface and in the upper part of the water. However at diving depths near the bottom the current was flowing south south west. This happens when the wind carries the upper water against a gentle tide in the opposite direction. A bit like the undercurrent in surf.