Tafsir Ibn Abbas: Al-Kahf, Ayah 61

So when they reached a juncture between the two between the two seas they forgot their fish — Joshua forgot his luggage leaving it behind at the moment of departure and Moses forgot to remind him — and so it the fish made its way into the sea that is it formed it — through God’s forming it — by burrowing that is like through a burrow which is a passage that is long and enclosed; this was so because God exalted be He held back the flow of the water preventing it from engulfing the fish and it the water withdrew from around it the fish and remained thus like a cleft without closing and what was beneath it the cleft stood still.