User blog comment:Xilinoc/Ch. 542 - THE BLADE IS ME/@comment-4207271-20130620040906/@comment-1970092-20130620123554

Oetsu stated that it was the immense heat AND reiatsu of Ichigo and his Zangetsu that evaporated that entire ocean/sea. As for the vapor, that seems like a negligible detail. I think it's more about the fact that he did it and no one has seen it before.