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Any owner of Xperia Z3+ here? Does the capless usb work well?
If it's as good or better than the SmartBand's waterproof usb port then it should be fine as long as you follow this
http://support.sonymobile.com/global-en/xperiaz3plus/userguide/Charging-your-device/
Hi,
The Z3+ has yet not been launched in the market I am in (Sweden), but I think the capless USB port will work just as swift and pleasing as it does on M4 Aqua and Z4 Tablet. 
The capless USB port will probably do its job. I dont think its better or worse but for sure safer. You dont need to worry about closing 2 flaps but only 1. However you might not be able to charge your device after submerging it in water, you may need to wait until it dries out.