Some TMNT stuff really isn't for little kids.
In the After the Bomb supplemental book Mutants Down Under, a fictional future version of the Great Barrier Reef is mentioned as one of the hazards that eventually made ocean-going travel impossible in the world of After the Bomb.
After an increasing number of ships started disappearing off Australia's east coast, it became apparent that some kind of unseen mutant life was living in and around the Great Barrier Reef, violently attacking those wandering into their territory. Some ships traveling in the area would even return to port, emptied of their crews, and sometimes stained with blood.
Initially Jakarta was suspected of disrupting ocean travel, but their ships started disappearing, too. The danger to ship travel spread beyond the Great Barrier Reef until it was no longer possible to travel by ship anywhere, effectively almost completely isolating Australia by sea.
Only the Black-Sail Ships traditionally maintained by the Buygees, a pirate people, could still travel the open ocean, their ships built in the old-fashioned way with light wood and sails. But the Buygees' remained secretive about where they were sailing and why.
However, travel over the ocean remained possible through increasing reliance on gas-filled airships.