you can use navionics, however depending where you are on the globe and where the sun is, you may not be able to see the screen unless its over-cast.
I have given up on GPS apps in Florida because you cant drive a boat while trying to shield the phone from the sun with your hat while trying to see the screen. not to mention in direct sunlight many phones and ipads will shut-down because they over-heat.
You can do that (download maps if you’re going to be offline) with Google Maps, too, and maybe Mapquest, too. Pretty nifty feature! I imagine most mapping software, nautical or otherwise, has a feature now to download maps.
I'm using an Ellipsis 7 tablet from Verizon and Navionics. It works OK but I need to get my cigarette lighter working because the battery doesn't last long. I ordered a bracket with a suction cup that I attach to the windshield. I keep the bimini top up and glare hasn't been a problem. I eventually want to get a Garmin but this setup works OK for me.