There are a lot of tunnels and secret caves which you can’t really see from a bird’s eye view, I’m thinking of adding a couple more islands before saying I’m done with it. Each island is themed, for example, one is a desert, one is a volcano, one is a swamp etc. The large rocky one in the middle is the one you start off in.
Love the use of a variety of different terrain types and techniques, one thing I noticed is all the beaches are very flat, maybe try adding a bit of a slope to them. A useful technique for sea floors is just a large sand brick, it blends pretty well with the sand terrain and saves on many voxels. Top of the rocks feel a bit flat too, maybe adding some large mountains for more elevation?