Navionics. It's the same cartography used in their product that you can install on just about every chart plotter these days. I use it a lot for passage planning on the big boat I run, I can then wizz routes/waypoints over to the onboard Raymarine system by wifi.
Also handy for planning Searider adventures, and it's nice to know I've always got a backup means of navigation in my pocket eg my phone. And yes, it will work everywhere (regardless of cellular range) - see the attached screen shot mid Pacific! (Note only ipads with a sim card slot have GPS built in)
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