Getting married in Japan

Hello everyone
We are wanting to get married (just us two) in Okinawa, Japan, and travel across from Tokyo to Osaka. We thought September would be good as its not peak season but it seems its peak Typhoon time then. Has anyone been to Japan around September and what was the weather like?

Also is it easy to legalise the marriage there? I'd read that we just need an affadavit to sign at the office over there, but also that its not always recommended. Quite confused whether it would be more of a pain to do but MOH really doesn't want to do a registry office thing here in the UK and then also have a ceremony over there which I understand and would rather have itdone just in japan and it be legal and we're properly married by the end of it.