These are all the gray anchors on the map - here is the list of locations:
Gelato Island (missable)
Mushroom Island (missable)
Matchstick Island (missable)
The ones marked “missable” just indicates there is nothing in the main story line to take the character to these places to unlock. Also, the places on the map under clouds are another indicator to what has not been discovered yet.
NOTE: I experienced a lag of this achievement unlocking for me to where I had logged out to the title screen and back in and the achievement then popped. The achievement did not pop on the last docking port reached in the game. If you experience the same thing, quit out to the title screen and come back into the game. This is a glitchy achievement.