So since you are saying that the best swimmers rose to the top and that the fossils of these creatures are found in the shallowest rock layers, why aren't these rock layers full of nothing but fish fossils? Surely fish must swim better than any other creatures, right? Or weren't the fish killed in the flood (contrary to what the Bible says)?
Furthermore, according to your "theory" all of the large creatures should be found in the upper rock layers and all of the smaller ones in the lower rock layers. We have found ape-like fossils in relatively shallow layers and various dinosaur fossils in lower layers. Aren't dinosaurs larger than apes?
No. I figure the aquatic critters would more likely be found at the bottom, but not necessarily. Creatures made to inhabit the lower parts of the earth are more likely to be found there. But with forty days and forty nights of torrential rain unlike the earth has ever seen or will ever see again what should one expect? Normalcy WRT to the laws of physics, but irregularity in the manner and degree of their application. The result? Both overall consistency and irregularities, just like the fossil record.