[brudgers]

The Challenger flight path was approximately vertical and so the debris field was relatively close to shore and in shallow waters of the continental shelf.

Given what the documentary crew was up to, it seems probable the debris showed up on sonar and the dive team went down to check it out.

So it is not exactly a chance discovery in the deep Atlantic by tourists on holiday. It is well funded professionals discovering something that looks a lot like what they were looking for.