Recently I stumbled across MX Mariner, a nifty chart plotter app for Android phones and tablets. This is well worth checking out, and it's only $7 in the Android app store.
While pretty new and still under active development, MX Mariner is better than nearly every other marine charting app in the Android store - and certainly nothing can beat it at this price. Many charting apps are impractical for real use as they require a live and fast data connection, they require saving charts one-by-one in advance, or they don't support chart quilting at all. With MX Mariner, you download a region when you have a good wifi connection - these are big files, 150-250MB, but once you have the file for a region, MX Mariner will automatically quilt charts for that reason as you navigate - and no data connection is required! Note that switching to Google Earth view does require an active data connection, but since the primary point of this is for marine charts, that's reasonable.
Some features, like GPX import/export are still under development, expect to see new features soon! Like OpenCPN, MX Mariner embraces iterative development, and you can even contribute if you so desire. If you notice some similarities between OpenCPN and MX Mariner, that's no accident - as the primary developer of MX Mariner is also a contributor of OpenCPN.
Currently, MX Mariner has somewhat limited world coverage, take a look at the website for more details.
So, if you are looking for an Android based chart plotter app, give this one a try! It works great on our Samsung Galaxy-S phone, and HTC Flyer 7" tablet. Looking forward to see what features come next!