Claiming rules
1.
yYou must have a wall around your land except the water borders
2.
To claim anything you have to build a building (fully player built and consisting of atleast a 100 blocks 100 block tall towers dont count) and then you can claim anything in a radius of 50 blocks, any other builds count too
3.
If you want to claim an island then just build things atleast 50 blocks from the coast as stated in rule 2
If the whole coastline is in the radius of your buildings then you can claim the whole island