No H2O has not been canceled at all. They put season 3 out last year and there are rumors saying tht h2o season 3 is coming to America in July. No idk if thts true but its possible! :)There filming season 4 and i kno this cuz claire holt confirmed it (AKA Emma) on her youtube channel. if you wanna see season 3 heres the link 2 a youtube channel where you can find all 26 ep. of season 3. plz enjoy and i hope i answered all of your questions. good luck watching the episodes. :)! <3 link
no its not cancelled at all its not showing because they are still filming i know it might sound weird but they are just filming and editing it before show if you want to see the whole seasons go on to youtube and look up h2o just add water season 3 and there you go!