Is there an estimate as to how much of the oceans total volume is occupied by living things? This includes micro-organisms, fishes, corals and any other living things in the oceans?
Is it significant enough that removal of all life from the seas would cause a drop in sea levels? Is it possible explosive growth of marine life could increase see levels?

I think the answer is that even though there is a vast amount of life in Earth's oceans, the volume is insignificant compared to the water. I don't believe changes in sea level would be observed with either sudden removal or expansion.