Actually that is the one unreachable area I fixed just now. It's a bug.
Just grab the new version from patreon that should fix it.
253 days ago(+1)
also the teleportation crystal that is on the island teleport unlock the mermaid city but the problem is that when you select to teleport there you can't go back because there is no crystal to go back
253 days ago (+1)
also if you use the dive ability to get out of water you can't use the swiming ability to get out of the water it only happen when you dive before using swiming