Phone replaceable battery; a rubber gasket and 3-4 screws - waterproof up to 5-6ft.

Though I guess you could get away with a cheaper clip based approach... Like some cheap Chinese ham radios. I had a friend drop one in a lake, then pull it out (maybe 3ft deep). You wanna know what part broke? The speaker - it was a bit muffled, but that's due to probably water penetration and pressure on that membrane.

It really is shocking how many people think it's "hard" to waterproof a small battery if it can be removed. Tupperware is water proof, doesn't cost a lot to make. Why would a battery encased in plastic be any harder? Especially for multi-billion dollar companies who can afford to scoop up some of the best engineers and have them work on consumer devices.