The Tidefall is a permanent POI in Fisch that can be a little tricky to reach if you are a new player. The main reason for this is a hidden secret pressure system that will decrease rapidly and instantly kill you if you don't follow the correct directions. So, if you are constantly getting killed or do not know the correct directions to reach Tidefall, here is a simple guide for you to follow.
Where is the Tidefall Entrance in Fisch?

The entrance to the Tidefall Underwater Cave is at GPS (3138, 132, 910). Here, you will find two boats with a diver NPC on one of them. Interacting with the NPC is not necessary, but if you do, he will explain about a secret opening underneath the Ocean.
How to enter Tidefall in Fisch
Before entering the Underwater Cave Location, you must have two things: Advanced Diving Gear and Tidebreaker. Yes, you need both items to reach Tidefall. The Advanced Diving Gear will provide you with the necessary amount of oxygen, while the Tidebreaker enables you to move faster inside the Ocean.
You can purchase the Advanced Diving Gear for 15,000C$ at the Buoy behind the Sunstone at (-790, 125, -3100). The Tide Breaker is at Desolate Deep and sells for 40,000C$.

Once you have both items, dive into the Ocean and head straight towards the stuck submarine at the bottom of the Underground Ocean. Just next to the submarine, you will find a small opening.

Go through it to find yourself in a small area with a rock to the right and a stream of water to the left. Touch this stream of water, and the force will automatically pull you inside the Tidefall. Voila! You have arrived in the mysterious world of Tidefall.
Fisch Tidefall Location FAQs
No, you cannot enter Tidefall without a Tidebreaker, as the default swim speed using even the best flippers is very slow, and you won't be able to even reach the submarine at the bottom without it.
The Purple Pressure Timer depletes extremely quickly, and you can survive only 10 to 12 seconds before it reaches zero, killing you instantly.
No, you cannot reach Tidefall with your submarine, as it can only reach a specific limit and cannot pass through the small opening at the bottom adjacent to the stuck submarine.
For more on Fisch, check out All 8 Divine Secret Fish in Fisch (Trade Plaza Update) or Fisch Trade Plaza Guide - Location, Booth & Trading