In each case, he descends--God made man--"goes down" and humbles Himself for the sake of His creation.
The little figure at the bottom of the icon of Christ's baptism is probably symbolic of the "dragon" spoken about in Psalm 74:12-13--Yet you, God, are my king from of old, winning victories throughout the earth.You stirred up the sea in your might; you smashed the heads of the dragons on the waters.
In the story of Jonah and the whale, we see another instance of water being connected to death and new life. This passage is used in the Orthodox Vespers for Theophany (The Baptism of the Lord).
For You had cast me into the deep into the heart of the seas and the current engulfed me. All Your breakers and billows passed over me. So I said, 'I have been expelled from Your sight. Nevertheless I will look again toward Your holy temple.' Water encompassed me to the point of death. The great deep engulfed me. Weeds were wrapped around my head. I descended to the roots of the mountains. The earth with its bars was around me forever. But You have brought my life from the pit, Oh Lord my God. While I was fainting away I remembered the Lord, and my prayer came to You into Your holy temple. ~Jonah 2:3-7Also, I heard on the radio yesterday that in Christ's case, Baptism doesn't sanctify Him. By descending into the Jordan, He santictifies the waters and makes them efficacious for our own cleansing, not only in Baptism but in its sacramental use as holy water.
Christ descending into the darkness of the water points, symbolically, to His releasing the souls from the abode of the dead at His resurrection, and releasing our souls from the sin we commit when we "do the very thing (we) hate". It also points to the need for each of us to die to self, and rise from the now sanctified waters, a new person, which can only occur with God's help.