I saw you wanted some critique on this.
Your shapes are fairly good overall, especially in this sort of composition. Some things I noticed is that one of the ears is not placed correctly, and that (at least imo) a drawing like this could be improved in a bit more subtle muscle details like around the shoulders, but that may be my preference.
The thing that sticks out the most is the background, I think you really could improve on it just by using more references and seeing how a beach looks on them. The texture on the water sticks out like a sore thumb and I think you have the skills to make it way better if you wanted to easily, especially seeing as you are selling it.
Other than that, I think you could also improve your shading technique overall, as the shadows here do not actually make all that much sense. I understand it is meant to highlight the butt, but it sticks out that you kind of shaded the right side of the character, with the light coming from the left, without considering cast shadows properly.
While I am harping on all the flaws, your character art & form is very good and I hope this did not discourage you in any way.