I have not used it in the salt but I have the boating HD app from Navionics.
It costs something per year (I don't recall) but I think it is worth it. You have maps (showing your position) with navigation, depth, and some structure information. It allows you to click (press) on a point and then navigate to it.
For offline use you can move the map to an area and then download the maps for that area. I have really good maps on my FF (and that is my primary) but I have used the Navionics map on the water when I am trying to find something far away (scroll and zoom are faster).
I am sure there are free GPS apps but I would be careful since free apps might require a data connection to serve the ads that allow for it to be free.