The shark is swimming backwards. Sharks cannot swim backwards, their pec fins wont allow it, and they can't just glide along, if they stop swimming they sink.
This is what I've used, you may need to adjust it for the length of his swim.
create seq brdfly loop,move 6 0 0 time=6 wait=2 ltm sync,rotate 0 15 0 time=2 wait=6 loop