With much respect, and without intention to unmotivate you from your creativity, I think you're doing something way too large for your level of pixel art. I think you should do smaller pieces first and hone on your skills such as shading, lineart, AA. Right now you have an idea, but your lack of skills is holding you back. The shading isn't right, and some pillowshaded. I don't understand the dithered outlines and also don't think the yellow outlines fit at all, or make any sense.
Also, I think the idea, while good, needs to be refined some more first. I see you've taken things like the planet and turned it into an island, and did the same thing with the ship/dolphin... but then you also have sharks and shrimp(?) in space as well, which is completely contrasting what you've done with the rest of the piece.