﴿ فَلَمَّا بَلَغَا مَجْمَعَ بَيْنِهِمَا نَسِيَا حُوتَهُمَا فَٱتَّخَذَ سَبِيلَهُۥ فِى ٱلْبَحْرِ سَرَبًۭا ﴾

“But when they reached the junction between the two [seas], they forgot all about their fish, and it took its way into the sea and disappeared from sight.”

Please check ayah 18:65 for complete tafsir.