فَلَمَّا بَلَغَا مَجْمَعَ بَيْنِهِمَا نَسِيَا حُوْتَهُمَا فَاتَّخَذَ سَبِيْلَهٗ فِى الْبَحْرِ سَرَبًا ( الكهف: ٦١ )
Falammaa balaghaa majma'a bainihimaa nasiyaa hootahumaa fattakhaza sabeelahoo fil bahri sarabaa (al-Kahf 18:61)
But when they reached the junction between them, they forgot their fish, and it took its course into the sea, slipping away. (Al-Kahf  : 61)
1 Mufti Taqi Usmani
So, when they reached the meeting point of the two seas, they forgot their fish, and it made its way into the sea as in a tunnel.