Al-Kahf 18:61

Falamma balagha majmaAAabaynihima nasiya hootahuma fattakhathasabeelahu fee albahri saraba

But when they reached the junction between them, they forgot their fish, and it took its course into the sea, slipping away.

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.