!!!!Possible spoilers ahead!!!!
How great is this settlement though. An entire island to make your own; and its bigger than any other settlement i've found
To those that dont know what this is its an island to the east of castle (little bit SE) that can be made into a settlement. To unlock the workstation all you have to do is go to the boat at the SE (I believe) side of the island. its the ship with the power lines running to it. There is a generator that you activate inside the first floor of the boat (and the luck bobblehead on the 2nd floor).
After activating the generator a mirelurk queen and many other mirelurks will spawn to prevent you from your goal. Just defeat all of them and head to the shack at the center of the island. This is where the workstation is located and to the left of the workstation is a switch. Just flip the switch (spawning a few more mirelurks) and the settlement is yours.
Sorry if this topic has already been made 100 times just wanted to make sure the knowledge is out there. Im loving this island.
Here's a clip showing how big the island is.
http://xboxdvr.com/gamer/XxCorey117xX/video/12830972
Alternate Method (and better method) courtesy of Chode Logan: You don't have to fight any mirelurks if you dont want to. Just avoid them and when you hit the switch in the middle shack with the workstation it will cause them to flee from the island.