The Flood - Animals and their minimum food requirement

Easy. 99.999999% of aquatic life lives only within a narrow range of salinity and light conditions. But those appropriate salinity and light conditions vary widely. With the entire globe a sheet of water, there wouldn't be anywhere near enough variation. Basically, you kill all the freshwater organisms or you kill all the saltwater organisms. (Itr's really more complex than that, e.g. the coral can't survive the depth, but that's the basic story).

Oh, no problem. A Few Silly Flaws In Walter Brown's Hydroplate Theory. Now, the problem is keeping it in the Solar Syatem and getting it back to Earth. Oh, and it probably took the whole atmosphere with it, by entrainment. But we'll just direct God to do a few more miracles, no worries.