My sweetie and I both installed
Navionics on our iThings to use as backups for cruises this past summer (Lake Michigan & the Pacific NW). It was about $40 - the most I've ever spent on an app. I found it extremely intuitive, easy to use, and very accurate. It also has access to tide data and can create/measure routes. It only took me a few minutes to download
charts over a wi-fi connection. I downloaded another set almost as quickly over a 3G network. It's also easy to set display units (E.g. depth and speed).
While this might seem obvious, a key feature is its ability to download and store maps on your
handheld device. The significance is that once you download the
charts, you have the map data even if you're not in cellphone range. (Many of the places we visit have no cell coverage.)
But it's just an app- and while it doesn't provide things like AIS, it's still a pretty amazing tool.