Why I don't use a computer for navigation is easy they crash and are far more un-reliable on a boat than a plotter! My Garmin & Raymarine plotters have never once let me down. Since I started using a dedicated chartplotter I've logged over 33,000 nm and except for one lightning strike they have been as reliable as a sextant for me. My laptops on the other hand have not been so good. I was running the Captain software and had nothing but problems with my laptop... Plus I like having a display at my helm. In 25 foot vis Maine fog you can't be running back and forth to the nav station!!!
The stylus on most tablets are propietary and cost about $30.00 to $70.00 to replace. In reference to an above post about Fujitsu tablets being more reliable they are slightly more reliable tnah HP's but less so than the IBM's. My wife uses Fujitsu tablets in her practice and she's on her third in 16 months. I went through two HP's in about 9 months & I now use an IBM. So I'd say they are about equal in terms of reliability. When you try and build a unit to the lightest weight specs they break and fail. A boat is a much more harsh environment than a medical office so I still say stay away from tablets.
You can buy a Garmin plotter with all US waters pre-loaded for $500.00 clams. Even the cheapest laptop pc is $599.00 plus the software & external GPS receiver.. Boat stuff is NOT always more expensive it's just USUALLY more expensive. A plotter is much more durable than a laptop on a boat period even if it's a tough book. Oh yean and none of the tablets we own is daylight visible even under the bimini when compared to either my back up Garmin or my Raymarine C-80.....