So the Lord said, “I will blot out man whom I have created from the face of the land, man and animals and creeping things and birds of the heavens” (Gen 6:7).
A common objection to the flood account is that Noah's ark wasn't big enough to house all the animals, and even if it was big enough, eight passengers are hardly enough to care for them. A dilemma.
That's not an issue for old-earthers, who espouse the local flood theory. From their standpoint, the ark only contained a representative sample of regional species.
For young-earthers, it's more complicated. Of course, young-earthers have standard answers to standard objections.
One issue concerns the scope of Gen 6:7. The wording seems to describe land animals. It's a shorthand for classifications in Gen 1, although it tellingly omits to include aquatic creatures.
Whether insects were included is a tricky question. But let's skip that for now.
One question is where to draw the line between land animals and aquatic creatures. That's because we have borderline cases. In terms of their degree of adaptation to an aquatic or terrestrial environment, animals range along a continuum. A shark, dolphin, orca, and octopus (to name a few) is totally adapted to an aquatic environment.
However, a sea krait is more adapted to an aquatic environment than a water moccasin or anaconda, but less adapted to an aquatic environment than a sea snake, while a horned viper is a land animal, although it can swim in a pinch (I assume).
A seal or crocodile is more adapted to an aquatic environment than the beaver, otter, or hippopotamus, but less adapted than a shark, dolphin, orca, or octopus, while a mink or raccoon is less adapted to an aquatic environment than a beaver, otter, or hippopotamus, but better adapted than a marten.
The tortoise is a land animal while a turtle is aquatic. Most crabs are aquatic, yet there are land crabs. Some frogs are primarily aquatic while other frogs are primarily terrestrial or even arboreal.
For the young-earther, it's an interesting question which animals would be included or excluded from the ark when it comes to borderline cases. Of course, young-earthers deny a one-to-once correspondence between prediluvian and postdiluvian species. I'm using modern-day examples to illustrate a point. Presumably, there'd be prediluvian analogues (i.e. borderline cases).
Rather than a taxonomic distinction, a young-earther could draw a pragmatic distinction. Which animals made the cut would be a question of which animals could survive (or not) outside the ark.