I think maybe you should work with tiles instead of sketching out and filling in this large of a background, so I'd scrap everything besides the refined left part of the sketch and work from there. The stuff you laid in in the back isn't going to stay in any fashion, so it's just distracting.
The underwater part is a neat idea, but you aren't really adding anything new to it that the last person who made a mockup like this had. The highlights on the grass strands you have right now need to move more to the yellow side of the color chart, while the darker parts need to be much bluer.
The health bar in the corner looks fine to me.
Overall I feel you're sticking to closely to what has already been made; bring some of your own stuff to the mix, whatever it may be.