Welcome Bimini. As a graduate student in Physical Oceanography, I enjoy this topic and the ensuing conversations. What exactly do you mean be 'killing'? What scientific evidence are you using to back up this claim? Is it all the oceans or just a portion? I think if you clarify your question and put some supporting material to your question we can have a better discussion here. Since your question had the word 'who' it sounds like you've already decided it must be mankind or a subset of. There may be some 'what' involved as well. Long before man arrived in his present form, during the Permian Mass Extinction, nearly all the ocean species died off. Yet here we are today with all that we have. So where do we begin? Do we want to discuss a particular area of concern or just make generalizing statements of how bad we are?
Or, perhaps more philosophical discussion. What if all is going along to a master plan? Let us assume past extinctions happened for remarkably good reasons. We might not have all the species diversity we have today without them. In fact, we ourselves might not be here today without them. If we or other geologic events are killing our oceans, how do we know that is not a good thing. Perhaps part of the big game plan is for man to starve himself out (or blow himself to bits) in order to make way for the next thing.