This isn't a game mockup, it's a triglyceride overdose.

You need GUI for it to be a mock up... (graphical user interface) like health bars and such... Maybe create a sea monster or something...and if you say you arnt good at creating monsters and stuff...Try and make some crates or some form of point system on screen.. Like in Sonic it was rings. Something like that.

Also try not to use to many colors. Old school games tend to be very simple color wise. You do nice work. I think this is a pretty peaceful little scene. But try not to rush your work. Notice where the water meets the shore..There are 5 pixels that are solid white. Dont forget to look over your pictures before submiting them.