you've got a nice start here; kudos for drawings something out of your normal routine. I really like the clouds and the little tree. The ledge near the top is interesting and gives the island a more fantastical feel.

The main thing that really throws me off is that there is nothing to distinguish between the sky and the water. They're the exact same color and tone. The water will be a bit more green while the sky will be blue. As the water moves away from the island, it will get deeper and therefore darker (and darker than the sky). Right now the waves in the water aren't very convincing, but those can be hard to pull off. Here are two wave tutorials; they use different techniques and the results are different, but in both you can tell you're looking at water with some disturbance.
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[link]

The line where the grass meets the sand is really abrupt and too smooth I think. Adding some of those trees along the grass line will help with the transition.

The waterfalls could be a bit transparent. In some spots I can't tell if it's part of the river or if it's actually a waterfall. the river portions could be a bit darker, too. Some perspective on the rivers will help give some depth (as the river gets closer to the bottom, it will appear wider since it's closer to the viewer).

It would also help to add some rocks. The island most likely won't be entirely covered in vegetation--there will be some boulders here and there and rock outcrop.

hope this helps!