Nobody has stated that the instructor was following the guidlines stated. I have seen some classes at the lake that I would not consider "by the book" I have also seen instructors leave students due to the fact she was to cold to finish the dive.
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One of the things is to ensure the students do come back within the specified distance. Sure this can somewhat be done from the surface by bubble watching, but I like to actually set up underwater points of reference and see what the students do when they don't make it back to the starting point. During the last dive of the navigation course, I also set up a course with headings. The students start at the first marker, head in the direction noted on the marker, get their next heading at marker 2 and head in that direction, and so on. Can't do this without being in the water to set the markers. Besides, I just like diving. I became an instructor because I like to dive, not because I like to watch bubbles. Even if it's just a navigation course in 25 feet of water, it's still diving.