InavX for navigation
Tides for tides and currents
Wunderground for forecasts, radar, and current conditions from both buoys and weather stations.
I want to be clear when I say that I don't think it's safe replacement for a good chartplotter.
I'm not going to bash any particular software, but, I will say that I have tried a few chartplotter applications side by side with a simple Garmin GPSMAP and found that ALL the apps have small issues.
Some lost signal, some put you in the wrong place then quickly correct themselves, etc. I do not trust them as much as I trust a "real" GPS.
Now, there are those that will say that this should not happen and throw all kinds of technical jargon out to explain why. There are those that will say that the smartphone GPS is not the same as a "real" GPS and toss around just as much jargon. I can't argue with either. I'm just throwing out my own opinion based on very unscientific tests under very real conditions.
Again, I love the navigation apps, but, I can't trust them (yet) from what I've seen.