These are the Boundareefs of the map, where the designers decided to swap the Invisible Wall for this more elegant if geographically implausible frontier.
In Real Life, you'll very rarely (if ever) see such conspicuously perfect lines of rocks leaving no single opening for going further, but these are just so convenient. After all, the options to create a very clear frontier in the open ocean are quite limited — there is little Chokepoint Geography that can be adapted to the marine environment.
Sometimes, Boundareefs can be used to delimit clear corridors instead of creating an artifical boundary to the map.
It is noteworthy that this trope can cause some Fridge Logic issues that Invisible Walls do not: namely if there is mention of sailing ships or old shipwrecks within the enclosed space, especially when it is mentioned that they come from far away. How on Earth could they get past the reefs?
- Hype: The Time Quest provides an interesting variation, for there are mountain cliffs that enclave the players' roaming in the sky. That's right, the region of Torras is enclaved within a ring of rocks that are higher than the clouds.
- The Legend of Zelda:
- The lack of boundareefs on the edge of the The Legend of Zelda: Phantom Hourglass map doesn't mean the game averts this trope: there are many conspicuously aligned reefs placed through the map to create wide corridors forming an "N" on the world map. Also, several islands are almost completely encircled by obvious Boundareefs, most glaringly the Isle of Ruins: there is only one opening in the ring surrounding it, which is conveniently blocked by a Cyclone that must be disspiated with a Plot Coupon.
- The Lanayru Sand Sea in The Legend of Zelda: Skyward Sword seems big enough but its explorable part is entirely enclaved by a thin strand of rocks. It seems the Sandship did not have a whole lot to explore after all.
- The Ryoshima Coast in Ōkami is surrounded by some rocky formations that prevent Orca (and later Amaterasu when she has access to the Water Tablet) from going further into the ocean. However, this brings a question: how did the Sunken Ship get past them?
- StarTropics loves this trope. Early in the game you get access to a submarine to sail from one island to the next in, but you're more or less forced through a linear sequence because each island is bounded by reefs that prevent you from just going around them. Each chapter invovles you having to explore the islands to find ways to clear a path for your submarine to move to the next patch of "open" ocean. Very late in the game, you have more freedom to submerge and find passages past the reefs underwater but it's still not nearly as open as the ocean should be.
- In Golden Sun: The Lost Age, the oceans are open for exploration, but landing on the previous game's continents or going up some rivers is prevented by big spiky reefs. There are also flat reef circles in the middle of the ocean that can be used to make landfall, but they only come in useful after the ship is upgraded to a Global Airship, which can then go in the middle of the reefs to look for the sunken items there.
- In Mario & Luigi: Superstar Saga, the source of the page image, the Oho Ocean is limited by a thin but impenetrable line of rocks, preventing Mario and Luigi from surfing too far.
- Pokémon games are huge fans of this trope. Wherever the player might go surfing around, there will be boundareefs to delimit the edges of the explorable sea. This is particularly notable because the reefs in question take the form of thin, precise and very closely spaced natural fences, forming clear corridors in the sea that all happen to coincide exactly with the main routes through the games' regions and to perfectly enclose and connect the various coastal cities' harbors, without a single missing or superfluous rock anywhere. This wasn't averted until the DLC areas in Sword & Shield, which don't even bother with this trope and simply put invisible walls off the coasts, with the water graphic giving the implication the waters in the open oceans past that point are too rough to be biked (yes, biked) on.
- In Chrono Cross, the El Nido archipelago is almost entirely encased within a rock boundary. There is one gap in the rocks — with an inward-flowing current. It's said that during low tides the current dies down enough to allow travel out of the archipelago, but this doesn't happen during the course of the game.
- Donkey Kong Country 3: Dixie Kong's Double Trouble! features these in a few early on places. The game has you traverse the overworld map in a variety of seafaring vessels, and the beginning motorboat can't cross the reefs. After obtaining a Plot Coupon in the form of a giant bandaid, you gain the hovercraft and you can skip these. But the hovercraft is of course stopped by yet another barrier it can't cross, etc. etc.
- The Bubble Breeze Galaxy in Super Mario Galaxy takes place on a planet covered with a sea of poison. The explorable areas (and them only, the rest of what we see of the planet is devoid of rocks) are some sort of reef labyrinths in which Mario must navigate while Floating in a Bubble.
- Sunless Sea: Subverted with the northern edge of the Neath: while there are countless icy rocks standing in the way north in an impossibly straight line, leaving only thin gaps between them, careful navigating can sail boats past them. However, you will soon really regret those rocks didn't stop you from going North.