AP =/= health bar. At least in one piece. WB on his deathbed destroyed Marineford.
No, no, the island was still there, being used.
The last thing he did was to partially depart 30% of the island. After that, the island was still usable.
I'm not saying that Wb is not able to sink an island, I even argue that he was being restricted, but none of his attacks in MF with the exception of the tidal waves and the air grab, had Ap at island level, he never used one attack at that level in combat during MF.