Genesis 7:21 Every living thing that moved on land perished—birds, livestock, wild animals, all the creatures that swarm over the earth, and all mankind. 22Everything on dry land that had the breath of life in its nostrils died. 23 Every living thing on the face of the earth was wiped out; people and animals and the creatures that move along the ground and the birds were wiped from the earth. Only Noah was left, and those with him in the ark.
Did the fish just carry on?
Gen 7:20-23 says this (with my emphasis):
21 And every living thing that moved upon the earth perished—birds, livestock, animals, every creature that swarms upon the earth [, and all mankind. 22 Of all that was on dry land, everything that had the breath of life in its nostrils died. 23 And every living thing on the face of the earth was destroyed—man and livestock, crawling creatures and birds of the air; they were blotted out from the earth, and only Noah and those with him in the ark remained.
The following words (highlighted above) are used:
Thus the author of Genesis uses three different words to denote the habitat of creatures that were destroyed, namely creatures that live on dry ground that breathe.
That is, the fish are not mentioned as being destroyed because fish do not breathe and do not live on the land. Thus, fish we unaffected by the flood.
Note that these same three words appear in Gen 1 creation account for which the main point is the creation of arable dry ground for cultivation (See especially Gen 1:10). The flood was the undoing of the creation account.
If all terrestrial plants were covered in water, this would have led to drastic changes to the earth's atmosphere (it would be equivalent to destroying all the terrestrial plants all at once), which in turn would have led to changes in the ocean. (Think slightly increasing atmospheric %s of CO2 are a problem for sea life? That would be nothing compared to this.) Further, rapid admixture of water from somewhere else capable of causing a complete, worldwide submersion of land very well would have changed the salinity of the water, resulting in problems for various marine animals and vegetation. Any fresh bodies of water would be submerged and mixed with the oceans, leading to widespread death for animals adapted to fresh water.
If we are to take the narrative in a 'literal' sense, it is highly unlikely fish would have just 'carried on'. More likely, the Biblical narrative of Noah's flood reflects an ancestral memory in Jewish tradition of a large, localized flood, or perhaps a global flood that was highly disruptive to human civilization but didn't actually submerge all of the land.
Addendum: if you want an answer sympathetic to Biblical 'literalism', this site attempts to answer the question from that perspective. I believe it understates the problems aquatic life would face from this sort of event, but they conclude that
"Many aquatic creatures were killed in the Flood because of the turbidity of the water and changes in salinities and temperatures. Indeed, the geologic record testifies to the massive destruction of marine life, with shallow-water marine invertebrates alone accounting for an estimated 95 percent by number of the fossil record."
