There are a lot of sharp edges/lines creating sharp contrasts between light and dark in the rock formations behind the building structure. I think it would look more natural if these were remedied.
The edge of the waterline is quite distinct as well, this could be softened (and the waterline made more varied), as the water would be thinning out slowly as it extends out onto the beach. The gradation from dark blue to very light blue/transparent water would likely be more gradual than you have depicted.
Watch your symmetry on the building. One side is slightly larger than the other.
Using photo reference will help your paintings become more believable.
I think this is a very creative fantasy landscape! Nice work