Abyssal Tonic is the single most important item you need to enter Tidefall safely, and the game intentionally makes you earn it rather than simply buying it at the start. You cannot walk into Tidefall without it because underwater pressure will slowly drain your health and eventually kill you.
Your journey begins at the Underwater Opening somewhere in the open Ocean. Near this entrance you will see broken boats and Deep Diver NPCs sitting around as if they have been stranded for a long time.
One of these divers will give you a quest called Tidefall: Lost Diver. This is not optional, it is the official gateway into the update.
To complete it, you must dive down and locate a lost diver lying deep beneath the surface. He appears as a small brown figure resting on the ocean floor, usually beneath the Tidefall entrance area.
Once you find him, interact with him to rescue him, then swim back to the surface before your oxygen runs out.
After returning to the quest giver, you are rewarded with your first Abyssal Tonic. This item temporarily protects you from pressure damage underwater.
To use it, simply drink it before diving toward Tidefall. The effect lasts long enough for you to reach the cave entrance safely.
Later, once you are inside Tidefall, you can buy more Abyssal Tonics from the central hub for C$500, which makes re-entry much easier.
Abyssal Tonic is best used together with Advanced Diving Gear and Super Flippers, because movement and oxygen management still matter even with pressure protection.
Without this item, Tidefall is effectively locked, so treat it as your official key to the entire update.