Does anyone know what is that big dark spot in the water?

That would be the mysterious underwater cave known as Sealed Chamber. You can only get there by goind through a certain spot in the current. Otherwise the current will pull you away from it, just as you're about to reach it.

And make sure you have a Pokemon that knows Dive, because once you get to the dark spot, use dive, and then you'll go underwater. Continue along the underwater path, and you'll find some Braille writing in the wall. Use dig there, and you'll find yourself in the Sealed Chamber. I won't tell you anymore - you must unlock the secrets of the Secret Chamber yourself.

Save the game at Pacifdlog then keep trying to get to the place with the dark spot. The rocks are there to make it difficult. So just try and find the right bit that leads straight into the dark spot. That spot is where you dive, it leads you to a sealed chamber Go to the entrance and then dive to the surface, it will take you to a room with graves in. Crack the code then get the Pokemon: Regirock, Registeel and Regice.

Thanks xxx

It is a dive spot if you dive there you will find a brail puzzle.If you complete the puzzle you will unlock the entrances to the Regis caves.There you will have to unlock other puzzle to find three pokémon that are rare.