/diy/ - Do It Yourself

How would you go about waterproofing a small device with a charging port? I want to make a water-resistant (lets say it needs to survive submerging in a cup of tap water) electronic device, with at least one port to charge it (preferably micro USB, as it will contain digispark, but it can be pretty much anything). The water-resistance should be as reliable as possible, so no flaps. How did Sony made their Xperia Z line phones submersible while having no flaps on audio jack, an from Z5 not even on micro USB?