Wow, I really like this question! Very little of the ocean floor is older than 125 million years old, because the ocean floor get continually recycled after getting dragged down (being subducted) into the mantle. So we can get relatively young fossils (young in geological terms of course!) from the ocean floor, like spores, pollen, mammals, spiders and also ammonites and dinousaurs! But we are actually better off looking for fossils on continents if we want older fossils! But the deepest parts of the oceans are fascinating places to study living animals, especially at hydrothermal vents, the places where new ocean crust is created. Lots of funky creatures live in the deep ocean! Check out the pictures here: http://ocean.nationalgeographic.com/ocean/photos/deep-sea-creatures/
Hope this answers your question!