My Samsung ATIV 700 T convertible tablet/laptop is pretty close to ideal as a boat computer for in-cabin use. It runs Windows 8 Pro, so I can use it like a full laptop on the nav table and run any software that runs under Windows, and I can detach the keyboard and it works well as a tablet (under 2lbs). Disk can be encrypted. At home or office it can be hooked up to (up to 4) large external monitor(s) at 1920x1080 resolution and keyboard/mouse, so I'm just using the unit as a CPU. It supports full USB 3.0, HDMI, Bluetooth, WiFi, micro SD card slot, and has an internal 128GB SSD hard-disk which is faster, lower power and more reliable than spinning disk. Runs for 5+ hours on battery, too, using an Intel i5 processor -- so it's fast. For connectivity aboard we use a Verizon MiFi 4G which also connects the iPad for online services.

I don't see the need to be untethered. I use Coastal Explorer on that machine at the nav station, and it overlays AIS info on the charts (which it receives via USB from my AIS B transponder). At some point perhaps I might set up a WiFi or Bluetooth bridge to NMEA 2000 so it would get AIS without wires, and also connect the Raymarine instruments, but don't really see the need for it.

A loose computer or tablet in the cockpit is not going to have a long life. It needs to be mounted and protected from drops, being sat-on, water and direct sun. Dedicated CPs are rugged enough to be in the sun and salt spray, so they will continue to have their place even though they're not as flexible.