Why does it say "you came to the sea"?
Radak: Yehoshua was addressing elders who left Egypt. 1
The decree to die in the Midbar did not apply to Leviyim, men below 20 (or above 60) at the first census, and women. (PF)
Why is Keri'as Yam Suf mentioned?
Malbim: Hashem wanted to add more miracles, lest people doubt His ability and Hashgachah