When I was playing around in survival, I noticed that when a creeper blasted upon me even though in water, the terrain was damaged.

Way to recreate : Create a new infinite world in creative, search for a "pond" and spawn a creeper in it. After spawning, ignite it with flint and steel. In some cases you will see it damages terrain, while sometimes it doesn't.

CONFIRMED IN 0.15.7
Also confirmed in previous versions. (Of course)

Way To RECREATE: Spawn in a new creative world.> Find a water body where there is land beside. There may be 2-3 blocks gap from the land.> Summon a Creeper (in the water) with the spawn egg.> Press and hold the Creeper with a flint_and_steel.> Observe the damage done by the creeper.
Note: 1-The creeper only damages blocks above water, not underwater blocks. 2- In the PC version, the creeper doesn't damage blocks above water or below water if ignited/blasts in water.