BigEasy,
I work for Navionics and may be able to help you. First, as a moderator on other sites and viewer here (I own a SeaCraft) I am sensative to this and should state: I may work for Navionics but you will get honest, non biased responses from me, first I'm a user, second an employee. If you or anyone feels otherwise in what I state, I expect to hear about it. Given I represent a commercial entity I want all to know my intentions are genuine.
You can find out more about them here:
Navionics -
http://www.navionics.com/en/mobile-pc-app
Cmap -
http://ww1.jeppesen.com/marine/light...ces-charts.jsp
You asked a good question that comes up often at boat shows etc. First, is it realistic to map on a tablet? The short answer, yes. But, I think what you mean is, is a tablet going to replace my Cmap plotter on my boat? The answer is not likely. Tablets are great devices, but lack a lot of the functions your plotter has to offer. First and foremost, they are not ruggidized and not terribly visible in the sun. Can it be used as a compliment for tracking or planning or back up device? Absolutely, and they are great for that.
You should also know that Cmap has thier own app, Plan 2 Nav. Some people like Navionics, some like Cmap. Depending on how old your plotter and mapping is, you may like the Cmap user experience better because it more closely resembles your Cmap plotter. In general, I would not expect to replace your plotter with it, certainly won't replace radar or sonar. I use Navionics, Cmap, Garmin, EarthNC, INavX all quite a bit and there are pros and cons to all of them.
Purchasing either can be done through the google play store for Android devices. Purchasing is the most complicated part about them, than you Google...
The pricing is more or less similar, Navionics gets it up front, Cmap gets it when you buy the map coverage and there are some differences in the coverages offered between the two.
In general, both will plan routes, waypoints, track your position with a breadcrumb trail etc. similar to how your plotter operates. I use my iPhone all the time, and an iPad very often on my boat. The phone I track every trip I take and review the tracks in google earth. I have several years worth at this point which is helpful come fishing season.
If you are interested in Lakes, Navionics has an advantage here with significant inland lake coverage and 1 ft contours on surveyed lakes. If you want a compass feature built in, Cmap has an advantage here.
Navionics has some add-on functions that allow you better routing control with the Nav Module as well as some shading controls via the Advanced Map Options. These are available in the Upgrades menu within the app and we continue to improve these all the time.
With weather, both apps off tides and currents. Cmap has an arrow for currents that I like, but I like the usability of the Navionics tables better.
Navionics has grib files for wind directions from NOAA but the one that has the best weather within a navigation app is Garmin in my opinion. But, its pricey! They have sat weather overlaid with the chart but you will pay for it.
Instead, I use Radarscope, NOAA buoy data, The Weather Channel, Fishweather (my favorite) or Intelecast which is also not cheap for an app. I am not sure if they all have android compatibility yet.
If you have any specific questions I can probably help, or can post examples of screen shots etc.