Hi dandan,
Rinsing with bottled water probably does very little good since most infections of the external auditory canal are caused by resident bacteria that have already adhered to the lining of the ear and can't simply be flushed away like that.
As regards the DAN material that MikeS has supplied, the first page is not of use as Otic Domeboro® Solution has been off the market for years, and no similar prescription medication has taken it place. Too bad, as that stuff was great. Go to the second page, entitled "More on Swimmer's Ear" for info on mixing your own preventative.
Best regards.
DocVikingo