I wrote to Sailproof yesterday; here's their reply in full:
Thank you for your message and interest.
Yes, you can indeed use Navionics on Android. In fact, most navigation apps are available for Android as well. Regarding Navionics specifically, you can use the same login across Android and iOS devices. However, please note that since this year, Navionics allows only two devices per account for simultaneous use (please confirm if this applies in your case). This setup lets you use our rugged, waterproof tablet outdoors, exposed to the elements and sunlight, while keeping your iPad or iPhone as a backup or for occasional use indoors. We don't offer any tablets on iOS as it is exclusively available for apple products.
If you have any further questions, we’re always happy to assist.
Now I am thinking about either a Panasonic Toughbook or a Sailproof tablet (and an Armor-X case for the current iPad is on its way). I had no idea that the one Navionics subscription could be run on both an Android and an iOS device at the same time. I use a charging block at the bridge currently, so the iPad can stay on while I am using.