I've got the Navionics on my Android phone and it's pretty good. I like it as a backup (I haven't installed a chartplotter on the new boat yet). The only highly annoying thing is that it doesn't suspend the screen blanking, so everytime I want to look at it I have to unlock my phone again. I think it was $15 or something for the whole of the US and Canada, and you can download areas for offline use (I have everywhere I am ever likely to cruise for the next several years at least). It's come in quite useful and my phone is waterproof too so I can actually use it on the boat without having to hide away or put a honking great case on it.
Also on a recent flight, my pilot was showing off his fancy GPS iPad application that had set him back a few hundred bucks. I pulled out my phone and my $15 Navionics and not only told him the inlet we were flying over, but how deep the water was