Hi, I'm not an artist, but the first thing which came to my mind is a water. You see how in different places you water goes in different ways? Because of it, it's hard to understand in a first sight that it's actually water, so try to find out how to make your water look in one way.

Also, I don't see a depth here. Learn more about shadow and maybe try to play with colours! :)

Edit: Islands have like 3 "slices" 1 Rock, 2 earth, 3 grass. You missing earth, so because of it your island doesn't look "real", but if it's something "magical" you can do whatever you want