Now I know that there's a whole new world to explore when an area glittering on the map.
Then you misunderstood, or maybe you are confused with a different game.
Like PiSketch said, it is just a hint of where you should go next, to help those players who have no clue what they should do and need a little bit of direction.
In this particular case, the author wanted you find someone swimming around at the indicated point. Both for game play and plot wise, it would be wise to recruit him before moving further to the right past the fish shaped submarine. If you had already done that, then you can ignore the signal.
The only fish-shaped submarine I know of is on the way to/from the turtle cave and the bubble cave you need beast form for. Been there, done that?
The only fish shaped submarine is in the veil, found after you barely entered the right hand veil area. There are no fish shaped submarines in the turtle or bubble cave.