"And when the seas are made to overflow," means: their salt water will be caused to flow into their fresh water, and their fresh water into their salt water, according to the interpretation of Qatadah.
﴿وَإِذا الْبحار فجرت﴾ فُجِّرَ مِلْحُهَا فِي عَذْبِهَا، وَعَذْبُهَا فِي مِلْحِهَا فِي تَفْسِيرِ قَتَادَةَ
