It's a striking scene, something I rarely seen in Digimon fan art.
MegaSeadramon's gigantic scale is presented well through composition — even without the ship being dwarfed, the low perspective doubled with his menacing appearance brings focus to the colossal size. Speaking of perspective, it's also nice that you made use of his long body to show him snaking in and out of the ocean.
Notably, the water obstructing nearly 1/4 of the view is clever as it both shows ferocity in the waves based on MegaSeadramon's activity and prevents the headache of figuring out how to paint waves splashing around his body as he rises out.
The detail is great! The scales shimmer, the neck creases like a serpent, and the shading in general is good. You even went the distance and gave the moon some texturing. The only point that sticks out is the lighting on his helmet: guessing you wanted to bring emphasis on his ominously glowing eyes — unfortunately I find the ambience too busy and unnatural. Also, it would of been great to see some visibility of MegaSeadramon's body underwater!
Otherwise, this has a lot of impact, and is one of the more original Digimon paintings I've seen.