The best way I've found for inputting Lat/Long is to use the search function in the Navionics app. To do it...
1. Click the magnifying glass in the top, left corner which will open the side bar.
2. In the title block of that new menu, there should be a button for Lat/Long (just above the search bar)
3. When you click Lat/Long it will change the menu to a screen
that you can input your values, do that then click "Show on Map"
4.It will show you the point on the map with a small title block above the point -- tap the title block to go to the edit screen
5. Tap "Marker" and you can add it as a waypoint
Hope that helps -- let me know if you'd like to see screen shots