This is just a personal preference but, I feel like the "under the sea" theme became a bit boring, I've seen too many maps with that theme.
Also another thing I noticed is the shark in the middle, it reminds me a lot of the map
@ValeHundred made for Team skywars, which got accepted, and the fact that the shark comes out of the sand looks a bit weird to me. Anyways I think the shark is well done and I like the fact of putting fornitures inside its mouth.
The starter islands and the sub-middle islands look a bit small and I think they need more details to make them look more interesting, they're almost empty. I also find the middle island a bit boring with that square shape.
I think the layout you used is a classic and maybe you should try to make a layout never seen before that can make the gameplay different.
Also, as I said before, the "under the sea" theme is a classic as well. I'd like to see new themes, you'd have more chances that your map will be accepted.