Crush
Contributor
You don't need to be a great swimmer to be a good diver, but you do need to have a high panic threshold in the water. Most people who are good swimmers also have a high panic threshold in the water, most people who are not good swimmers have a low panic threshold when in the water. That is the real issue. Experienced instructors know that swim tests are just a way to get a read on the potential student's in-water panic threshold.
Thalassamania, FWIW I agree with you entirely. However, I'd like to draw attention to one of my OW classmates. He was on the high school swim team. While that was some time ago, he was still a very good swimmer. However, when it came to (on a single breath) the 25 meter swim underwater (with fins), dive down, find your gear and put it on, he crashed and burned badly. He swam to the end of the pool on the first attempt, dived down, then bolted for the surface. Next, he made it 20 meters and abandoned his attempt. Repeat at intervals of 20 (again) and 15 meters.
Summary: While not being a strong swimmer often predisposes you to panicking underwater, sometimes being a strong swimmer is not enough to make you calm underwater.