Psalms 106:11

וַיְכַסּוּ מַיִם צָרֵיהֶם אֶחָד מֵהֶם לֹא נוֹתָר

The waters covered their adversaries; not one of them was left.

This verse is quoted in the brachos between Shema and Shemoneh Esrei. The Malbim explains that a tzar (translated here as “adversary”) is worse than an oyeiv (an enemy). A tzar is an oppressor. What the Egyptians did to the Jews in captivity far exceeded the mutual antagonism of enemies, which is why they deserved such a harsh punishment.