"Or it becomes" means: or it turns into "its water sinking deep" meaning: disappearing, having gone deep into the earth, "so you will never be able to find it."
﴿أَو يصبح﴾ يَعْنِي: أَوْ يَصِيرَ ﴿مَاؤُهَا غَوْرًا﴾ أَيْ: ذَاهِبًا قَدْ غَارَ فِي الْأَرْضِ ﴿فَلَنْ تَسْتَطِيعَ لَهُ طَلَبًا﴾.
