Most folks tend to overestimate current speed ... primarily, I think, because moving water is such a powerful force to deal with ...
Tacoma Narrows passage. Daily tidal currents typically peak at about 4 knots there, but on a big exchange they can approach 7 knots.
Although currents in the Narrows can reach those speeds, dive charters won't take you there on very large exchanges. A typical drift dive in the Tacoma Narrows is going to be between 2 and 3 knots of current. Anything over 3 knots and the dive charters are unlikely to put you in, due to concern over turbulance and downcurrents.
Here's today's Narrows currents ...
Currents at The Narrows (North End), Puget Sound, Washington
On a side note - is there any easy way or indicators to measure strength of current?
Rips mask off - 7
e.g. nearly but not ripping mask off = 6
Requires hard finning to remain stationery = 4
Could happily sleep here = 0
Just interested as discussion of current seems pretty subjective.
The strongest diver out there might ...
might ... be able to remain stationary by hard finning in 2 kts of current ... but even so, only for a very short period of time. For the average diver, that's going to be closer to 1 kt. I own an X-scooter ... which is a fairly powerful toy and can go way faster than I can swim. A couple weeks ago I did a dive where I had to get to an upline against the current. I was barely able to make headway with the scooter, and wouldn't have had a prayer of doing it without the scooter ... the boat captain was measuring the current against ground speed at about 2.5 knots.
Nusa Penida in Bali, Indonesia
That's the only place the current has pulled me so far from the boat that it took them almost a half-hour to find me ... and then only because another boat noticed us and told them where to look.
But hold on to a line in a 2 knot current and you will start falpping like a flag in the wind.
... ah, there we go ... that's much closer to my reality. Anything more than 2 knots and you're going to have trouble even holding on to that rope.
My strongest current was in Agate Pass (WA State). It was 4 knots where we got in and 2 knots where we got out. At 4 knots you can't make any progress even if you are kicking hard and are perpendicular to the current. At 2 knots you can swim diagonally and gradually make some progress.
My strongest was also Agate Pass. The boat captain told us it hit 6 knots, and was just silly strong. We didn't see much. First off, it was like watching a movie in fast forward, and secondly because we shot through the pass in about 20 minutes. At one point I grabbed ahold of a rock on the bottom to try to stop myself and it bloody near took the glove off my hand. There wasn't a prayer of hanging on to anything. The water flipped me around end-over-end instantly, and it took me a wild few seconds to get re-oriented and promise myself I'd never try that again. Swimming against it wasn't even a consideration ... it simply wasn't an option.
I can't even imagine what it must be like at 10 knots...don't turn your head is all I can say! I would imagine that you can't hold on to anything at much past 2 knots.
10 knots would be Deception Pass or Dodd's Narrows at their meanest. Every place I've ever seen with that kind of current have eddies and whirlpools that look like the product of a nuclear-powered washing machine. I wouldn't consider taking a boat through current like that ... much less diving in it.
Farnsworth bank, 3mi off the back of Catalina.
Entered 4knot current, exit 7-8+ knot current. The current had 60ft Kelp lying flat agents the reef. We used it like a rope to pull ourselves forward of our boat, then inflated our BC's and rode the Kelp almost to the surface. Once on the suffice the boat was about 100 yards to the side and 100 yards down current. I took off my rig and handed it to my dad for later pick up. I made a mad dash sideways swim for the boat, and barley caught the end of our 300ft drift line. Pulled my self back to the boat, grabbed the dingy and went to go get dad 1/4 mile down current.
If you could even hang onto a rope in 7 kts of current, you have the worlds strongest grip. To pull yourself 300 feet against it you'd need the stamina of a distance runner and the upper body strength of an NFL linebacker ... and it'd most likely undress you long before you made it to the boat.
... Bob (Grateful Diver)