I've got a iPhone SE, and I have a waterproof case for it. It fix it to the bike, I have an additional waterproof pouch, with a self adhesive 'quarter turn' mount stuck to it. The case has a lanyard that gives me extra security of clipping round the bars - I've rescued other cyclists phones from being run over after they lost them.
The touch screen still works through both waterproof cases at once - all cheap off ebay.
I have Viewranger, with the free 'roadmaps' map, but don't often use it as a 'sat nav'. I'm not sure it would have enough battery life, so it's there if I need it to navigate back to civilisation if I'm lost. I do use Strava with the screen off and it will run all day on my phone, and still leave 50% battery.