I've dived the Rhone 10+ times (one of the advantages of having my own compressor aboard) and never noticed a current or the state of the tide. I think that the divemaster in Kegoangoango's post might have been overestimating the water speed, a human with fins swims at about 2 knots and 6 knots of current would 3 times faster than a person can swim. While a river might get up to those speeds, the currents around the BVI don't. At 6 knots the Rhône itself might have moved And the water would have looked like soup.