A very promising sketch. Dynamism definitely is a strong point of your drawings.
I’m curious, though: is it 100% digital or scanned pencil ?
composition-wise, I don’t have much to say. The immediate focus of the picture is the gamping maw of the island-turtle, from which I immediately jump to the boat. Because you’ve made the turtle occupy so much of the screen, the man is really dwarfed by its hulking mass and we feel that he has no escape: the picture tells its story well.
It took me a while to notice the skulls & bones underwater, a kind of second-glance detail that I appreciate. They really add to the feeling that the place is dangerous and the man in the boat isn’t the first one surprised in this manner. However, if I were nitpicking I would notice how these bones suggest shallow waters since they’re so close to the surface, and subsequently wonder how such a big turtle could have found enough depth to hide its lower body. But it’s really far-fetched and can be easily solved by puting more distance between the bones and surface.
Last, one small critic about the hand study: the shading gives the impression of a weird swelling in the middle of the hand, as if the base knuckle of the thumb had been displaced. Instead, I would expect a bulging mass closer to the base of the hand