well it may be a question of labellign at the importation then... here, all "adult" anime is labelled hentai... and in fact if i follow the definition i was given once a while from the person who brought me into japanese animation (wholly, not only adult one) some of the elements found in hentai makes many series to fit in such label, i mean as expressed in the native land. but well it is not the place for that i gues.. i did not suggest you to broaden your anime knowledge, just the hentai part of it, but as you say that for you, not all adult animation is called hentai, well i won't argue... marine boy ? i guess the serie had another name in m coutnry too. I saw many different translation of series when brought abroad from japan. Some ever very weird or strange /irrelevant translations so it is sure hard to figure what serie you are referring. in fact for umetsu yasuomi (mezzo forte, kite, yellow star) in fact it is less hentai, it is closer to typical tv movie of police story and thriller. In fact if you look at his work, there is plot, and not much sex scene, maybe as few as in a regular tv movie (just a bit more displaying, not suggesive...) well anyway one thing i can swear i am sure is that, there (in japan) hentai does not always mean bondage or tentacles so far (though bondage or toys are a strong fetish in their sexual life and culture and is ought to be present), and that usually hentai is supposed to carry "humor". anyway well it is not the place as i said. maybe it would be interesting (for both of us) to try to find out a true definition from a native, ney ?