User blog comment:Pacifista15/Explosive Punches/@comment-3387274-20111031144605

I agree with the below. It was partly down to cavitation. My short answer is haki from the attack that the attacked caused a WB style shockwave that went through Hody, transfering the extreme heat from Luffy's arm. This also superheated the water into steam, and briefly displaced it all as well, causing a vaccume surounded by boiling water. The vaccume imploded, once the shockwave was gone, sucking in the superheated water, causing an apparent wave of explosions surrounded by steam. This would also work on land, but the added density of water made it worse.