Why did Hashem stop Moshe from crying out to Him?
Rashi #1: Because Yisrael were in trouble it was not the time for lengthy Tefilos.
Rashi #2: The salvation of Yisrael is not dependent on Moshe (or on anybody else) but on Hashem Himself. 1
Ramban: Moshe knew that the Egyptians were about to be destroyed, 2 and he was crying out to Hashem for advice on how Yisrael should proceed. Hence Hashem told him that there was no need to cry out, and that simply asking would have sufficed.
Seforno: When Yisrael prayed (Refer to 14:10:3:1), Moshe prayed too, and his Tefilah here was (not out of fear of the Egyptians 3 but), on account of the Resha'im, who brazenly asked about a shortage of graves, thinking that they would not listen to him to enter the sea. That is why Hashem ordered him to stop praying and to instruct Yisrael to enter the sea - and to see for himself whether the Resha'im would not obey him. 4
Targum Yonasan: Because the Tefilos of Yisrael preceded his. 5
Hadar Zekenim: [You need not pray, for] their (Bnei Yisrael's) merits and those of their Avos are enough to save them.
Lev Eliyahu p.82: It does not depend on Me, rather, on Yisrael. There is prosecution
As if the Torah had written "Alai (on Me) with an 'Ayin' (See Sifsei Chachamim). See also Yeshayah 45:11.
As Hashem had told him earlier (in Pasuk 4 [Ramban]).
Seeing as he had already informed Yisrael of their downfall - in Pasuk 13 [Seforno]).
To prove that he had been Choshed bi'Kesheirim (Seforno).
Since the Tefilos of a Tzibur are more affective than those of a Yachid.
How were they expected to travel, seeing as the Sea was in front of them?
Rashi: Based on the merits of their fathers and their own, and the faith that they displayed by going into the desert, 1 the sea would split before them.
It is not clear why Rashi does not include the merit of Mesiras Nefesh - the fact that Yisrael were about to comply with his command by moving forward even before the sea actually split.