Imagine you're on vacation in Greece. It's a nice sunny day, so you decide to go for a swim. The bottom of the sea is full of some greenish-brown ugly seaweed, every stone is covered by some fuzz and all around are scattered these things that look like poo and many more things.
That's why I decided to do something. I took control over the world and now I will destroy all life in the oceans! (Muhahahahahaaa!)
Just a little problem - I don't know how. I want to destroy all (or as much as possible) visible life of Earth's oceans in less than, let's say, 10 years
Bonus points for an answer:
Not destroying all life on Earth, leaving terrestrial life as untouched as possible (anyway it will slowly die out without the ocean life support)
Destroying even deep-ocean and bacterial life
Cleaning up after the wipeout and making the seas accessible to humans
Doing this as fast as possible
I thought of various methods, like poisoning the sea, but I'm not sure, what to do.