As Anne has found out, you need to download the charts for the area you will be traveling while you have high speed internet connection for the Navionics App on the iPad. That would be before you need them and do not have suitable connections.
The same can be said for the Garmin. Make sure the chip you have installed will give you coverage along your route. Garmin's web site has detailed maps with this coverage. You can also install the chip and search for a location along your projected route and see the detail of that area while you are still at the dock in Seattle, Anacortes or New Your City.
While the Garmin Blue Chart app has some great features, it has been so buggy and unreliable that is it not my app of choice for the iPad.