I have to rave about this app. I got it for my iPhone but I am sure you can get it for the miscellaneous brands as well.
I have heard good things about it in the past and after hearing Joe having good luck with it I had to spend the $10 to get it.
For $10 you can download every lake, river, ect water map for the U.S. It shows very detailed depth maps as well as channel markers and other marked Buoys. The nice thing is on a lake when you see a danger buoy in the middle of the lake it hard to know what is the danger area. With this app you can see it. It's amazing how accurate it is.
While you are driving the map moves and gives you a red line showing where you are going and a yellow of where you have been.
Even worked when I did not even have service as seen in the pics!!
Also lets you save your "track". Gives top speed, total distance, and time duration.