Why exploring the ocean is mankinds next giant leap

Because we've got space covered.

I'm joking. Sort of. We really don't know about the majority of the ocean as it is today, even with all of our fancy-smanshy cameras and submarines and stuff. There are a lot of trenches and deep-deep-deep places with fishies and things that we haven't even discovered. Some of the places we can't get to yet because technologies haven't been invented to allow humans to reach them. We know there are many more marine species to be found because new ones are discovered every year, perhaps every day, and there are species ranging from microscoping plankton to blue whales that live, breed, and die entirely in the water, so we might not even know they are out there. It is pretty exciting. There is a lot to learn.