I am with Nemrod and have said it many times in previous, similar threads.
Last Friday I went diving in Jupiter. Our 1st dive was on Deep Ledge. The current was extremely brisk and the swim down to the edge of the reef required strong, competent finning and good fitness. There were 5 of us with the DM, nobody was obviously unfit. Two of the divers were unable to make the swim, were seriously winded, and called the dive. Both of theses divers proved to be perfectly competent on the other 2 dives we did that day, they just weren't up to the requirements of the 1st dive.
I have many similar examples, good technique and fitness frequently make the difference between being able to swim around on the Castor or hanging on for dear life.