In this spot you can find legendary fish every 5 to 10 min. The requirments are lvl 20 fishing and legendary lure.
Here is my secret fishing spot where I find Legendary fish every 5-10 minutes.
Be in this spot exactly not around the corner. In this exact spot.
I've caught 4 out of 5 of the legendary, but I have seen all 5 in this spot multiple times.
My advice is to press space and reel in once the lure is halfway to the boat because I only catch legendary fish when the lure is as far and deep as possible. Also, I recommend you catch at least 3 of the big fishes at the bottom like sharks, swordfish, etc. because usually thatll speed it up. But leave the lure as far as possible so that a legendary can come if you keep catching big fish it won't come as fast.
In the pic above once the lure gets this far press space cause you won't see a legendary when its that close to the boat.
Here is my secret fishing spot where I find Legendary fish every 5-10 minutes.

Be in this spot exactly not around the corner. In this exact spot.
I've caught 4 out of 5 of the legendary, but I have seen all 5 in this spot multiple times.

My advice is to press space and reel in once the lure is halfway to the boat because I only catch legendary fish when the lure is as far and deep as possible. Also, I recommend you catch at least 3 of the big fishes at the bottom like sharks, swordfish, etc. because usually thatll speed it up. But leave the lure as far as possible so that a legendary can come if you keep catching big fish it won't come as fast.

In the pic above once the lure gets this far press space cause you won't see a legendary when its that close to the boat.