Hmm, free? A lot of us love Navionics with contours on our phone, but if that is more than you need, I can see what I can find.
Technically you could just use the Google maps app that comes with your phone. You would have to use your computer to set the way point, because the app tries to find a land based address when you try to add a spot.
You’d go to Google Maps and create your own personal map by clicking My Places button on the upper left. Then you can create a new map with markers. Then on your phone you can view them by selecting the My Maps layer and choosing the map.
Of course this won’t help if you want to set them on the water.