The Swim Suit unlocks swimming. The developer’s Wyrm’s Remains recording shows its pickup message explicitly explaining that Lief learns to swim, followed later by movement through the dungeon’s water channels.
Where the recorded pickup happens
Look for the room with a raised northern alcove, blue-and-green jars near its upper corners, and rows of flames on either side of the central floor. In the developer recording, a large orange enemy occupies the middle before the reward is collected.
After that encounter clears, the player moves north to the reward and obtains the Swim Suit at approximately 10:00–10:01. Room and pickup
This landmark description refers to the fiery room state shown in that recording. Wyrm’s Remains also has water and ice appearances; do not identify a room solely by its colour.
Check that the ability has unlocked
The pickup is the important confirmation. The developer later shows Lief travelling through blue water channels at 11:00 and again at 15:00. Those examples establish swimming after the Swim Suit pickup; the Fairy Slinger icon in the item slots is not itself evidence that the slinger grants swimming. First water example · Later water movement
If you are earlier in the game, a water obstacle does not mean you missed a swim button during the opening. The recorded equipment progression places this ability inside Wyrm’s Remains, well after the first-dungeon fairy tutorial.
What this unlock does not establish
The evidence confirms water traversal. It does not prove immunity to lava, purple hazards or every other floor effect. Continue reading the room’s obstacles after obtaining the suit rather than treating all dangerous-looking terrain as passable.
For the dungeon’s boss, continue to Fiend-Shen.
Source: developer footage. The pickup and swimming effect have not been independently corroborated; the route from the dungeon entrance is not covered.