I understand where you are coming from completely and have thoughts and experience to share but I would caution that a device on the dash is probably not going to make your boat dummy proof for others driving your boat. A new driver needs to learn to get their head out of the boat to stay aware of their surroundings. This past weekend I was fiddling with the tablet settings while using it as a chart plotter while trolling and it was distracting enough at 3 mph and I consider myself to be pretty experienced.
I have found using the tachometer as a guide is the easiest way for a new driver to maintain the right speed for a wakeboarder. A perfect pass or similar cruise control system is probably the best route to go as far as getting a good pull from a less experienced driver, perfect pass does not recommend their produce for my engine package (3.0) otherwise I would probably go that route. I am fortunate that I have a good freind who rides with me most of the time who drives the boat well.
I have run MX Mariner on an ACER Iconia A200 which is a 10" android tablet with gps. I have found that the screen brightness has to be cranked up all of the way in order to have good visibility in daylight which definitely affects battery life. I also found that if the tablet went to sleep it would rarely recover the GPS signal without being restarted. To avoid this problem I changed the sleep setting to 30 minutes of inactivity before going to sleep. I was only using the tablet as a chart plotter and not trying to run other instruments through it. I was infrequently touching the tablet because I was trolling and using the tablet to see contour lines and bottom features, but rarely needed to touch it for any reason, so it was falling asleep and losing the gps signal untill I changed the settings around. I had the tablet charging through a 12V outlet/usb adapter and with the brightness cranked up the charge was still going down, I believe the contacts in the outlet were dirty and this could be fixed.
Like you I believe there is a great potential to use a tablet to achieve what would otherwise require a small fortune in marine electronics to accomplish but I wouldn't rely on it heavily until you have all of the bugs worked out. I think the biggest challenge would be displaying all of the information you want at once or to be able to toggle through them easily without them missing a beat. For example if I was using the tablet to display a blue tooth based depth sounder, the tablet I was using wouldn't be able to display that and the navigation app at the same time. I would focus on using the system to augment your existing instruments rather than replace them.
Please post back with your results. I am particularly interested in knowing how the Nexus does and if you can switch apps without the navigation losing it's position. The Acer I have been playing around with is my parents, my wifi ipad does not have gps (which is really my wife's) and I am trying to decide whether to buy a blue tooth gps for the ipad or a reasonably priced android unit with a gps like the nexus 7.
I think there could definitely be a market for an app which integrates and displays all of this seemlessly and capable of expanding to include audio, ballast systems, and whatever else your heart desires.
