I have half a mind to write the following cheeky e-mail to this man:

はじみてぃやさい。わんねぇ、トマスどぅやる。 渡嘉敷島んかい 行ちゅん。ダイビングしぶさんど。 やまとぅくとぅばわからんしが、うちなーぐちてーげー分かとーん。 うんじょー、わからに？ ゆたしゃん うにげーしゃびら。

("Hello, my name is Thomas. I am going to Tokashiki island and would like to try diving. I cannot speak Japanese, but I can speak some Okinawan. Is this OK with you? Yoroshiku onegaishimasu.")

It's written in the local language (of Tokashiki island, that is). He'll understand, right?

(In seriousness: someone should just tell him how off-putting his phrasing is and that he should state that unfortunately he only speaks Japanese and so if you want to join his diving tours you need to have enough Japanese to understand them. I've met people like this and they often genuinely don't know how offensive they sound and will make the appropriate changes when you ask.)