Which sea did Hashem dry?
Rashi: The Yam Suf.
In which river will people walk?
Radak #1, Malbim: They walked through the Yarden (when entering Eretz Yisrael. Hashem made the water pile up and not flow, so Bnei Yisrael could walk on the floor of the river - Yehoshua Perek 3.)
Radak #2: This refers to the future [redemption] - "v'Henif Yado Al ha'Nahar Bayam Rucho v'Hikahu l'Shiv'ah Nechalim v'Hidrich ba'Ne'alim; v'Haysah Mesilah li'Sh'ar Amo Asher Yish'ar me'Ashur Ka'asher Haysah l'Yisrael b'Yom Aloso me'Eretz Mitzrayim" (Yeshayah 11:15-16).
What is the meaning of "Sham Nismechah Bo"?
Rashi: There, there was something that rejoiced it - the sea saw Hashem.
Radak: Also we will rejoice when He takes us out of this Galus.
Malbim: We rejoiced [when He dried the Yam Suf for us, and enabled us to walk through the Yarden], for we recognize that He oversees people and changes nature for them.
