I use an iPad all the time with the Navionics software installed. I use it adjacent to my normal chartplotter. I usually have one zoomed out and one zoomed in. When both are zoomed in, the positional accuracy and map quality is essentially the same. I mark out all the RCA's as "paths" on the iPad and that helps me stay out of the RCA's. I find it a lot easier to use the iPad for this than the chart plotter. I also have Navionics on my Android phone and that serves as a backup (as do my buddy's iPhone and iPad both of which also have Navionics on them). On smart phone (Android or iPhone) the GPS is built in. If you buy an iPad that is 3G/4G capable, a GPS will be built in. If you buy an iPad that is NOT 3G/4G capable, the gps is not built in and you then need an external GPS puck such as the "bad elf" mentioned above. The bad elf puck goes for about $100 which is the difference in price between a non 3G/4G iPad and the 3G/4G unit. Since the cost difference is the same, my advice is to buy the 3G/4G iPad as that GPS is built in and it's one less thing to worry about. Also, you can turn on a data plan if you so choose later on. We use a Ram mount to hold the iPad on the window and that works fine. You'll also need a 12v charger that puts out a reasonable current to the tablet. Often the standard phone chargers can't keep up with the iPad when it's on full time.
The downsides to the iPad relative to marine chart plotter is that the iPad is not marinized and thus can deal with getting wet. Inside a boat cabin this isn't really an issue but in an open boat you need to buy some waterproof case. The other downside is that the screen at full brightness is not as bright as a marine chart plotter at full brightness. Again this is more of an issue in an open boat than in an enclosed cabin. I don't have any experience with the Android tablets but given that Navionics on my Android phone is functionally equivalent to that on an iPhone or iPad, I imagine and Android tablet would be similar (just make sure it has a built in GPS). The other advantages of a tablet is that you can use it as a movie or music player and when you have connectivity (data plan or marina wireless) use it send email or get weather, swell and fishing reports. I also store pdfs of the fishing regs, the boat engine service manuals and other user/service manuals for the things on my boat. It does a lot more than a chart plotter but in bright light and wet conditions a proper marine chartplotter does the primary functions a little better. Nonetheless, if you have an enclosed cabin, a tablet is a perfectly useable replacement for a chart plotter.