I think they'd have a water bladder I think its called. Just like fire types have a bladder to created flames, though I guess the water could be reaction based like how Houndoom makes fire by shooting toxins that react with the air.
As far as realist pokemon go I look to these things:
here for realistic but not creepy pokemon
http://arvalis.deviantart.com/ (they did one of the squirtle line)
here for realistic pokedex entries, pretty much like a real encyclopedia: http://tvtropes.org/pmwiki/pmwiki.php/JustForFun/ThePokedexExtendedFanonEdition?from=Main.ThePokedexExtendedFanonEdition