Rashi on Exodus 15:4/רש"י על שמות ט״ו:ד׳

1ירה בים Onkelos translates this by שדי בימא: HE HATH CAST IN THE SEA. The word שדי is an expression for “casting”. And thus, also, it says, (Exodus 19:13) או ירה יירה which Onkelos renders by או אשתדאה ישתדי “or he shall certainly be thrown down” where the ת is used in both these words in place of (as marking) the Ithpael form, so that in these Targum words the root is שדי‎.

2ומבחר AND THE CHOICEST — This word is a noun of similar form to מֶרְכָּב and מִשְׁכָּב and מִקְרָא [קדש]‏.

3‎טבעו ARE SUNK — The root טבע always denotes sinking in slimy matter, as may be seen from (Psalms 69:3) “I am sunk (טבעתי) in deep mire”, and from (Jeremiah 38:6) “And Jeremiah sank (ויטבע) in the mire”. The fact that this expression is used teaches, therefore, that the sea became slime (cf. Rashi on 14:24) to requite them according to their own measure — because that they had made the Israelites work as slaves with slime and with bricks (Mekhilta).