Why isn’t the ocean fully explored? Why are certain seas black and others clear?

Why isn’t the ocean fully explored? Why are certain seas black and others clear? I selected this subject because, as humans, we are so willing to explore in murky waters without knowing what species may be there or have not yet been found. For hundreds of years, humans have been attempting to see what lies in the deep, unexplored seas below the sea level. It’s interesting to know that there’s species that are not extinct that have been observed and are surviving, with a one in a million chance of being spotted, is fascinating. I’ve never understood why beaches in Florida are slightly more colorful than beaches in Massachusetts, or why beaches in other regions of the world have superior ocean color.