Try Navionics USA -- only $15 (for iPhone) when I got it. Don't know current price. I like it. More usable than Garmin and has a killer auto-route function for quick route planning. Downside -- not ActiveCaptain data.

I also use Skipper but I hate it for navigation. I like it because it has ActiveCaptain and also uses NOAA Raster charts, which contain details vector charts leave out.

If you want to interface other instruments (AIS etc.) over Wi-Fi then iNavX would be the one.