The flood which soon strike the earth was a judgment on man. However, due to the nature of the judgment, everything on the face of the earth with the breath of life would also be destroyed. This, of course, included every animal living upon the earth. Thus God commanded Noah:
And of every living thing of all flesh, you shall bring two of every kind into the ark, to keep them alive with you; they shall be male and female. Of the birds after their kind, and of the animals after their kind, of every creeping thing of the ground after its kind, two of every kind will come to you to keep them alive. (Genesis 6:19-20)Just as Noah would ensure the survival of the human species, so each animal pair would ensure the survival of their species. Gathering every single species of animal upon the face of the earth would certainly have been a laborious task for Noah. However, it appears that God conducted this "gathering" Himself. In verse 20, God states that two of every kind will come to you. Noah did not go out looking for the animals, the animals came looking for him!
Fish were not a part of the diverse crew which boarded the ark. Aquatic life was excluded... for obvious reasons. This, however, is not the first passage in which Scripture makes a distinction between land life and water life. At the creation of the world, Adam gave names to all the cattle, and to the birds of the sky, and to every beast of the field (Genesis 2:20). But nowhere does it mention that Adam gave names to all aquatic life. Assuming that the pre-flood world did not invent the submarine, only in the modern era has man plummeted the depths of the ocean, and, as a result, discovered (and named) some of the strangest creatures of God's creation.
As Noah began constructing the ark, the creatures of the earth made their way to that salvic scene— they made their way in order that the things of earth might be preserved for future generations. For even after the flood had ended, man would still rule over the fish of the sea and over the birds of the sky and over the cattle and over all the earth, and over every creeping thing that creeps on the earth. (Genesis 1:26).
Did Noah Gather "Vegetarian" Food? - Genesis 6:21
God's Covenant With Noah: Promised - Genesis 6:18
State Of The World (2469 BC): 120 Years Before The Flood - Genesis 6:3
God's Ark Which Noah Built - Genesis 6:14
Noah Found Favor In The Eyes Of The LORD - Genesis 6:8