So I posted about this on the help forum, and they suggested I'd set my object details to high instead of medium. And that worked. I could see the sharks as well as turtles and dolphin swimming in the water. Unfortunately, my graphics card gave out and I had to set it back to medium. So now I've ordered a stronger graphics card and also more ram. My CPU should be sufficient, as I have an Intel 5. This game is costing me so much money.