Here, I purchased a complete set of UK charts for under £15. Their clarity, level of detail and accuracy are outstanding. The app/charts allow COG, SOG, compass direction, waypoint marking, route planning and more. I haven't yet had a chance to test in Arwen out in the sound yet but I'm looking forward to doing so.
Another fun app with a serious use is 'marine traffic'. This uses google maps and tracks every vessel offshore. Click on the boat icon and up comes its course, speed, track, a photo of it and vessel details. So cool! As I write now, nine fishing vessels, line astern are passing down the eastern side of he sound, heading for the Cattedown. The sailing vessel 'one sail' is heading across from Weymouth and is just passing through Wembury Bay, closing on the sound.
With an anchor watch app as well I think I have enough to be getting for the time being!