Thanks I will check out the links, I have a dedicated tablet for my boat, originally I ran Navionics on it as my primary chart plotter, but now I have my Garmin goodies, which of course now is also Navionics. I use it now mostly for Bluetooth transmission of music to my radio & speakers.
Roughly five years ago there was a small angry article in Pacific Yachting, the couple had gone to hole in the wall and had used Ports and Passes for their "timing" info to transit the nasty section. Unfortunately a minus became a plus or vise versa and when they began to head in they realized something was wrong. They tracked it down to an error in P&P. This article confirmed to me knowledge I already had, for critical areas especially the rapids always use more than one source.