If control of the targeted creature changes between the activating and resolution of the ability (ie, while it's on the stack), then the player who must control an Island as the ability resolves is the creature's new controller.
: Choose target attacking or blocking creature whose controller controls an Island. This creature deals 2 damage to that creature at end of combat. Activate only before the end of combat step.
You can't target a creature if its controller doesn't control an Island as you are activating the ability, and the ability won't resolve if the creature's controller doesn't control an Island as the ability resolves.
: Choose target attacking or blocking creature whose controller controls an Island. This creature deals 2 damage to that creature at end of combat. Activate only before the end of combat step.