It turns out there are two Mermaid’s Caves. Well, one, technically – in the Past and the Present. And holes you blow in walls in the past affect the cave in the present too. Sadly, I needed another key to get into the cave in the past (it’s the same lock, so why two keys?), so I had to find that first. Two caves means two maps and compasseseseses too.
There are a few four dimensional puzzles to contend with in this dungeon, which complicates things a little. Also, the mid-boss (who I’ve killed twice already in the Oracles games) appears very close to the end boss. After killing the mid-boss, I got the Mermaid Flippers, which let me swim underwater and into new areas. They also make me swim like a retard.
As expected, they’re needed to help kill the end boss, whom I hit about ten billion times, but he killed me. Stupid thing.