Why does it mention ruling over the sea?
Radak: Before it said that He rules over upper beings. Now it discusses the haughty and strong lower beings, among the awesome creations. It says Ge'us ha'Yam, for the water is above the land, like I explained above (refer to 33:7:1:2). Water's nature is to leave and go on the land. Hashem rules over it, and stops it, and put sand to be a border for it.
Malbim: It explains how the two conducts (Chesed and Emunah) are revealed in the lower world. Regarding wondrous conduct, "b'Sod Kedoshim" (verse 8), here it refers to Keri'as Yam Suf. The sea was raised and passed its border when Hashem sent an east wind the entire night. During a storm, the sea is wont to pass its limit. You ruled over it when its waves rose - "uv'Ru'ach Apecha Ne'ermu Mayim" (Shemos 15:8). Via this itself, You quieted it - "Nitzvu Chemi Ned Nozelim Kaf'u Sehomos b'Lev Yam" (ibid). You made the sea dry!
What is "b'So Galav"?
Rashi: It is the rising of its waves. Radak - this is like bi'Nso, just the first letter of the root (Nun) is omitted.
What is the meaning of "Atah Seshabchem"?
Rashi: You lower them. The same applies to "v'Chacham b'Achor Yeshabechenah" (Mishlei 29:11) and "Mashbi'ach She'on Yamim" (above, 65:8).
Radak: You quiet them. The same applies to "Mashbi'ach She'on Yamim" (65:8). Perhaps this refers to Keri'as Yam Suf. Or, it is at all times - "Yakem Se'arah li'Dmamah va'Yecheshu Galeihem" (107:29). Or, this refers to the first half of the verse. When waves rise, and it seems that they will leave the [border of] sand and go on the land, when it reaches the edge, You quiet it.
