I have this new Nokia phone with a replaceable battery. It came with the pin you can use to open it (probably costs a few cents to manufacture, but if I had plenty of time to spare, I could've made one with a sturdy piece of plastic and sandpaper). The "mechanism" that keeps the back panel is just a piece of rubber, probably the same kind you find on hermetically sealing food containers.
Also, very few people want their phones water-proof (what for?). And even if you really, really wanted that, you could like spend 5-10 Euro on a water-proof case.