The Xperia Z series has marked the high-end of Sony's smartphone line for years. The M5 sits somewhere lower on the food chain, but not much lower. You're looking at a 5-inch 1080p display, 3GB of RAM, a 21.5MP camera, plus both water and dust resistance.
Now you also get Android 6.0. Sony has begun pushing out an over-the-air update to this device. The build number is 30.2.A.0.100. Sony is starting with the E5603 variant, but all others are set to get the goods eventually, including the Dual.
The Xperia M5 is joined by its predecessor, the Xperia M4 Aqua. That phone, which launched earlier in 2015, has a 5-inch 720p display, 2GB of RAM, and a 13MP camera. The water and dust resistance was there as well. It launched with Android 5.0, and now it's getting Marshmallow too. The new build number is 26.3.A.0.131.
The M4 Aqua update has hit select regions. It's going out to both the Aqua and the Aqua Dual. If it hasn't hit your device yet, hopefully it'll only be a matter of time.