What fear of Hashem is discussed?
Malbim: There is natural fear, and fear of choice. Yir'as Hashem based on man's nature, via his intellect, to fear due to the Cause of causes, i.e. Hashem Himself. Additionally, one should be aroused to this fear due to fear of damage due to Hashem's punishment, recognizing that He oversees to reward or punish based on his actions. The first is arousal of the intellect; the latter is arousal of the Torah. Chovos ha'Levavos discusses this (chapter 3). Yirah is the former, fear of Hashem's exaltedness. Tachilu is fear of His punishment. Man's intellect is Klali (general) and Sugyi (of a class). Regarding his body and nature, it is Prati (personalized). His intellect is not due to his individuality; man in general has intellect. It is planted in man's intellect to fear Hashem Himself, for He is the first Cause of the created beings. This is Klali, in everything with intellect - men and angels, whether it has physicality (man) or not (angel). Matters such as envy and pride are Prati - some people feel them, and others do not. Based on intellect to fear Hashem, one should only do what is proper, even without intent for benefit. However, man's nature includes desires, Midos and natures, so sometimes he veers from Yir'as Hashem to fulfill the desire of his body and nature. Bodily desires are possible, not necessary; he need not fulfill them. They are Prati, and not Klali. There are many of them; they sprout from different natures in the body. E.g. one wants to rebel due to envy, hot lust, cruelty
What is the meaning of "Tachilu"?
Rashi: It is an expression of fear.
Radak: It is like "Chil ka'Yoledah" (6:24, i.e. trembling.)
Malbim: It is fear of His punishment. (Refer to 5:22:1:1.)
Why did He mention a border for the sea?
Rashi: The sea does not worry about punishment, nor anticipate reward, yet it fears Me and does not pass the border that I set for it.
Radak: This is one of two 1 witnesses 2 that you cannot contradict. (a) From the world - at the beginning of creation, water covered the entire land, for this was its nature. I said that the water should be gathered to one place, and the dry land will be seen. When the water gathered to one place, it was piled high. It should have descended and spread over the entire land, like initially! However, I told it "Ad Po Savo v'Lo Sosif" (Iyov 38:11). Hashem's saying is His will. I made sand a border for the sea. It is not a high wall to stand against the water. Further, the waves rage and clamor and come to the border as if they will pass it, like their nature. They cannot, because I withheld their nature.
Malbim: A supreme natural Chok (decree of Hashem) stops the sea from passing its natural Klali border. This is indispensable and does not change. Hashem made it to sustain the creation. Even so, the water, based on its nature, rages to transgress this Chok. Great waves are raised via wind, or gravity of the moon, or other reasons known to physicists. Even so, since these reasons are Prati, and dispensable, they submit to the Chok Klali. Even one wave does not pass the border! Therefore, it is proper that also man do so. Even though his nature aspires to transgress the Chok of intellect (fear of Hashem's grandeur), and his bodily desires rage like great waves, he must submit to the Chok Klali.
Radak: The second witness is among you. When you do My will, "I give to you your rains in their time" (Vayikra 26:4) - Yoreh in Cheshvan and Malkosh in Nisan. "V'Nasnah ha'Aretz Yevulah v'Etz ha'Sadeh Yiten Piryo" (ibid.) Rain will not fall at the time of harvest. "Shevu'os Chukos Katzir Yishmar Lanu" (verse 24). Since you were special among the nations via this Brachah, how did you not put to heart that I oversee you?! How did you deny Me, and say "Lo Hu" (verse 12)?!
Radak: Why didn't Hashem give for witnesses the wonders that are written, e.g. Keri'as Yam Suf, splitting the Yarden and other breaches of nature that are possible only via an overseer who can change nature according to His will? They could deny them, for they occurred only once. Those who were not there at the time can deny them, if they deny Torah. This witness - sand is a border for the sea - is permanent. It will never depart. You cannot deny it, for you see it with your eyes and hear it with your ears! You see the waves until the shore, and they do not pass the sand, which is the sea's border. You hear the waves' clamor, as if they want to pass everything and cover the [entire] land. Man cannot deny this!
Why does it say "Ya'avrenhu"?
Radak: The Nun is extra. Also "Kol ha'Yom Yevarachenhu" (Tehilim 72:15) has an extra Nun, and also Ya'avrunhu at the end of our verse.
Why does it say "Yisga'ashu"?
Rashi: The waves rage and go high to pass the border, but they cannot.
What is the meaning of "v'Hamu"?
Rashi: They make a great noise.
