How does this Pasuk fit into the context of the Parshah?
Rashi: With reference to the following Pasuk, it means that when 1 the horses of Par'oh entered the sea ... ', that is when Miryam took a drum in her hand ... '.
Ramban #1 (citing Ibn Ezra): It is an intrinsic part of the Shirah - inasmuch as it relates how Hashem made the water crash down on the Egyptians even whilst Yisrael were still walking along the dry sea-bed - a miracle within a miracle. In fact, they sang it together with the rest of the Shirah. 2
Ramban #2: It is telling us that they sang the Shirah immediately following the drowning of the Egyptians, not the next day or at a later date.
Ramban #3 and Seforno: It is to inform us that Yisrael sang the Shirah whilst they were still walking on dry land in the middle of the sea, and not after they had come up. 3
Rashi in Teshuvos (Si'man 251) explains either 'when' or 'because'.
The Ramban maintains that it is not written in the style of a Shirah or a prophecy.
Rambam, Hadar Zekenim: And that that was when Miryam took the drum and read out to the women the first Pasuk of the Shirah for them to repeat after Moshe and the men.
What are the ramifications of the Pasuk "U'Venei Yisrael Halchu ba'Yabashah be'Soch ha'Yam"?
Targum Yonasan: There, in the middle of the sea, there were springs of sweet water and fruit-trees, and vegetables and sweets on the sea-bed. 1
Interestingly, on the same words earlier in the Parshah (14:29), Targum Yonasan made no comment; nor does there seem to be anything in the Pasuk to indicate what he writes here. (EC)