1)
Why does the Torah write here that the Egyptians sunk like a stone, in 15:10 - like lead, and in 15:7 - like straw?
1.
Rashi: The Resha'im were tossed around like straw before they finally drowned; the Benonim, like a stone, whereas the 'Tzadikim' sunk immediately, like lead.
2.
Hadar Zekenim (in Pasuk 10): Others maintain that it was the other way round: The 'Tzadikim' were tossed aroound like straw - and lived lonmger - the Beinonim, like stone and the Resha'im sunk like lead - and died immediately.