I love 2D design myself, but I've worked more in pixel art more than this kind.
I notice that the sun is directly in the middle, but the shadows that are being cast are in different directions.
The coral in the water below for some reason have an extra faded shadow above them, guess a duplicated and lowered opacity layer.
The words warning on the sign are crooked compared to the actual plank they're wrote on.
You really missed a great opportunity to show off the landscape of the island underwater, which is bad considering the water is shown to be almost clear towards the top/mid.
Seaweed is typically not shaped like grass, it usually is a little more curvy as it extends to the tip and it thins out as it grows from the base.
The fish are a nice touch, but they're not really affected by the light source too much the deeper they go.
The skull on the boat looks completely unnecessary and if anything it distracts from the scenery.
My suggestions.
Fix the shading compared to the light source.
Explore creating the underwater backdrop a little more, you can even simplify it by using light to dark shadows the further away from the screen it is.
Look at some references of seaweed and improve your design.
Don't make the island end so abruptly, give it some small bumps and variety of landscape, looks like a hamburger bun floating in the water.
I've edited some parts to give you a guide encase I'm unclear.
That's a rough 1 minute mock-up to show you some of the points I was making, don't try to do it as quick and sloppy as I did.
You have a good thing, you just need some minor improvements here and there.
It's coming along nice though, just give it some more depth and you'll be hitting on gold.
Keep up the good work and you'll be making amazing designs in no time!