Everything you make here is aimed at a dive that happens after the course ends.
When you go back to your station in six months, a year, or five years, you repeat Dive 3 exactly: relocate from the documentation, re-occupy, re-capture, re-score. That time the interval is long enough that a difference might be real.
Comparable means the same place, the same standard, the same method. The station relocated to within the offset your documentation supports. Frames that met the capture standard. The frame laid in the same orientation. The same class list and point count. Scoring done the same way. Where any of that differs, you state the difference and narrow the comparison — a return visit is an audit of your own earlier record, and the rules are the ones you just read for somebody else's.
A difference smaller than the floor is not change. Beating it clears a floor — it does not settle the question.
The floor you leave with has two parts. One is derived from the table, for the point count you used, and it is the same for anyone who scores that many points: nothing smaller than it can be told from sampling. The other is what you observed on Dive 3 — one repeat, on one day, with one set of charts, probably a few hours apart — and it governs only when it is the larger. A real return meets everything that day did not contain: different water, different light, a new chart batch, a different camera, you a year older. The course does not turn those into a number, because there is no honest way to. You list them, you state the floor you cleared and by how much, and you let the reader weigh the rest. What you leave this course with is a floor with a date and a point count on it — not a detection limit good for every visit that follows.
A null result is a claim too. No change was detected is a statement about the reef only if it arrives with the floor — the smallest change you could have seen. Without that figure it is a statement about you.
And change is not health. A second visit can support a claim that something changed and by how much. It cannot support a verdict on the reef's condition, because a verdict needs a reference your record does not contain — a comparable site, a regional standard, a longer history. A reef can decline and still be in better shape than its neighbors; it can improve and still be in poor shape. What you hand over is what changed and by how much, offered to whoever holds the wider evidence. That is worth considerably more than a verdict your data cannot carry.