If your bathroom is upstairs and accessible from the loft then you could use an in-ceiling single-stereo speaker or a pair of standard in-ceiling speakers driven by a Sonos ZP120 (or SB equivalent with external amp) - the mains operated ZP/ZB & amplifier sits somewhere safe outside of the bathroom and you just run in the extra speaker cable and cut the speaker into the plasterboard.
This is one of my favourite features of my Sonos system, lying in the bath or taking a shower listening to my music without interruption from the rest of the family
Then again the S5 plays loud without distortion so you could do it the old-fashioned way and just turn the volume up in the bathroom.
On a different note, the older Sonos CR100 remote is water resistant (there are even vids on Youtube showing it working in a sink full of water!), far more likely to survive use in the bathroom where there is a risk of dropping it into the bath.
Finally if you want a weather-proof, wireless battery powered external speaker that sounds fantastic then take a look at the Soundcast OutCast. Itis not cheap although also no more expensive than decent external speakers with a suitable amplifier. While I use it with Sonos it has a transmitter with stereo audio input and iPod dock so can be used with any system that can give an analogue stereo output. Sound quality is really good (the big Outcast has an 8" bass driver for tight, deep bass) and the battery runs for upto 8 hours even when driven hard. I use it constantly when the weather is good. It does look a bit like an ivory Dalek when it sits on the patio but I don't mind how it looks when it sounds so good