Nonbasic lands lose any other land types and abilities they had. They gain the land type Island and gain the ability ": Add ."

If a nonbasic land has an ability that causes it to enter the battlefield tapped, it will lose that ability before it can apply. The same is also true of any other abilities that modify how a land enters the battlefield or apply "as" a land enters the battlefield, such as that of Vesuva or Cavern of Souls.

If Harbinger of the Seas loses its abilities, it continues to turn nonbasic lands into Islands. This is because effects that change subtypes are applied before considering effects that remove abilities, regardless of the order in which those effects started.

Harbinger of the Seas doesn't affect names or supertypes. It won't turn any land into a basic land or remove the legendary supertype from a legendary land, and the lands won't be named "Island."

If a nonbasic land has an ability that triggers "when" it enters the battlefield, it will lose that ability before it can trigger.