There is a term for this in fisheries science: shifting baselines. Each generation takes the state of the sea when they first saw it as the natural state, and measures decline from there. Everyone is measuring from a lower line than the people before them.

What the baseline actually was

The reefs in Cuba's Jardines de la Reina, protected for decades and barely fished, are the closest thing many divers will see to it. What is striking is not the coral. It is the fish: groupers the size of dogs, sharks as an ordinary sighting rather than an event, and enough large-bodied animals that the water feels occupied rather than decorated.

The first time you see that, every reef you previously called healthy reclassifies itself in your memory. It is a genuinely uncomfortable experience — a bit like reading good code after years of your own.

Why protection works so visibly

• Big animals come back last and matter most. Predators take years to reach size, so their presence is a record of how long an area has actually been left alone.
• Structure follows. Complex living coral needs grazers kept in balance, which needs predators, which needs time.
• Fishing pressure is the dominant variable. Not tourism, not sunscreen, not divers. Those matter, and they are second-order next to what is taken out.

What to do with the knowledge

Mostly: stop rating dive sites on colour, and be suspicious of your own sense of normal. Ask operators what the site looked like twenty years ago. Where marine reserves exist, pay to dive them — the fees are frequently what funds the enforcement.

The film below is Cuba, and it is worth watching if only to recalibrate what abundance is supposed to look like.