Island Exploration Cannot Be Completed

ATTENTION! THIS MAY BE A MAP SPOILER!

I have been trying to complete this island with the Aztec Temple. However, I found that coordinate [R 18] cannot be explored even I had explored the surrounding pieces of the map.
The following is the picture of the problem.

 

If I could figure how to post a picture I could show you mine, which is unlocked. Just keep going around that island, it'll appear for you.

 

Do you mean I need to unlock all other parts of the island first?

 

Oh I just figure it out. Thanks for your reminding.

 

You'll unlock it once you defeat the ghost ship

 

Also, please use the tag [ spoiler] [ /spoiler] before and after the image in order to prevent spoiling the locations for others

 

I'm in the same position and I defeated the Ghost Ship already. He appeared in the exact same spot again so I went about finishing out the island. Got 2 hours left and if that didn't work I was going to post here. Let's see if that does indeed work.

I had figured that was how it worked but I never had this happen before. There was an island to the left that let me unlock an additional tile I couldn't reach when I explored the adjacent one and still had more of it to explore, which made me question why R18 was still locked.

 

Get the area around the island and internal squares that are locked will unlock.

 

Confirmed: It unlocked on the last voyage on the last tile. That little tile was irking me for about a week now.

 

Yea...I went through the same OCD issue with that...There are 3 or 4 like that.