First of all I'd like to say I adore how you've drawn little Vivi!
You've picked out the details of his clothes very well, particularly on the shoes and the jacket. You have a very good understanding of light direction and shadows and use them well to direct the eyes to the hand and spell. The light seems a little too over exposed in places and the glow on the gems could be a little softer - but - I must say the glow on the gems on his jacket are lovely and add to the water effect.
The main bit of advice I would give for this piece is about the pose. It appears as though you've wanted to have Vivi leaning into a strike however he looks like he is possibly falling backwards (please correct me if I'm wrong). Tightening up the pose would really add to the dynamic effect of the piece and really strengthen the motion of the spell.
Overall you've created a very lovely drawing of Vivi. I hope art block doesn't plague you in the future as you have a good eye for detail and thank you for sharing it.