main island
Refers to the principal island of a group, such as Okinawa's main island or Japan's Honshu in context.
Okinawa's main island is the island where Naha Airport is located.
The typhoon is approaching the main island.
this island
Used when the speaker is on the island they are referring to; context-dependent and less common than この島 in casual speech.
The population of this island is about 100,000.
Okinawa's main island is the largest island in Okinawa Prefecture.
We crossed over to the main island in a small fishing boat.