This illustration is an original concept that I realized for a contest launched by Spartan ( digital-painting.school ) during my training.
My first step was the realization of the background :

Digital Painting Carte Recifs Requins 01 Lady Melusine

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I changed the corals en route.

Digital Painting Carte Recifs Requins 03 Lady Melusine

Digital Painting Carte Recifs Requins 04 Lady Melusine
Then I drew the sharks.

Digital Painting Carte Recifs Requins 05 Lady Melusine

Digital Painting Carte Recifs Requins 06 Lady Melusine

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I added some light effects.

Digital Painting Carte Recifs Requins 08 Lady Melusine