Water resistance was my first thought as well, however I suspect a company with the R&D budget of Apple could likely find a way. The small dimensions of the watch might be a problem, but for phones I'm fairly certain it can be done.
For a while companies like Motorola were selling nano-repellent (essentially hydrophobic) coating phones and calling them splash proof. You could easily design a gasket or something, existing waterproof phones already have parts that open for eg the sim card or microsd slot (which are gasketed).