In part 3 of him answering science questions he is asked how much shallower the sea would be if we take out all animals. He answeres less than youd think and follows this with the explanation that fish are mostly made out of water. While i agree that its probably not going to be much displacement - many waters compared to little maritime life - the notion that fish are basically "all water" doesnt matter since water displaces water. Its about volume isnt it ?