Marine worlds

In order to improve my technics and workflow on painting environments, I asked Mister Paweł Latkowski if I can use some of his "Beneath the waves challenge" thumbnails to help me understand the composition and try to paint them.
You can find his submission and thumbnails here : https://www.artstation.com/contests/beneath-the-waves/challenges/32/submissions/24532

Any tips to progress are welcome :)