In short Yes, I believe it is.
"The Sony Xperia Z3+ has an impressive IP65/IP68 rating, which makes this dual SIM smartphone both waterproof* and resistant to the tiniest dust particles....
IP (Ingress Protection) rating
The Xperia Z3+ has been given the IP rating IP65/68. This is a standard that measures a device’s resistance levels to dust (6) and water (5&8 – both splashes and submersion)....
*The Xperia Z3+ is waterproof and protected against dust, so don’t worry if you get caught in the rain or want to wash off dirt under a tap, but remember: all ports and attached covers should be firmly closed. You should not: put the device completely underwater; or expose it to seawater, salt water, chlorinated water or liquids such as drinks. Abuse and improper use of device will invalidate warranty. The device has Ingress Protection rating IP65/68. For more info seewww.sonymobile.com/waterproof. Note the Xperia has a capless USB port to connect and charge. The USB port needs to be completely dry before charging."
http://www.sonymobile.com/global-en/products/phones/xperia-z3-plus/features/#waterproof