This Japanese language post has a link to information (again, in Japanese) about renting a sea kayak in the Amakusa area. A phone number is listed so if you or a friend can speak Japanese and are interested in sea kayaking in the Amakusa area, you might want to give them a call.
https://www.facebook.com/permalink.php?story_fbid=1919257411457407&id=212870155429483&__tn__=-R
– Kirk