The included lanyard is NOT long enough to fit around a neck of any size person or animal. You would want to just buy a separate lanyard that would be big enough.
The device is listed with an IP68 rating, and this is what that means according to Samsung: "Devices backed by an international standard rating of IP68 are deemed fit enough to withstand dust, dirt and sand, and are resistant to submersion up to a maximum depth of 1.5m underwater for up to thirty minutes."
That tells me it should be okay to use it in the shower.