Board Thread:Questions and Answers/@comment-13497263-20130726193119/@comment-11051098-20131108200559

It's not been revealed what Engetsu is. Its release command is "Burn", but that doesn't mean that it has to be flame-based.

The water in the SS dried up because of the intense heat caused by Zanka no Tachi. Ôetsu Nimaiya said that the sea by his palace dried up because of heat and reiatsu.

We don't know if Zangetsu is flame-based, only Kubo can answer that.