I have used MxMariner, NutiCharts Lite, and MyNOAACharts on both my android phone and tablet. MXMariner uses the phone or tablet GPS receiver and lets you download free NOAA charts regions for offline use and also can incorporate Active Captain information. Provides a good backup to your chart plotter.
The new Navionics app for IOS lets you download and use the free NOAA charts in offline mode. The android version of the Navionics app does not allow this yet. You still have to buy their charts.