Please note: all credit goes to Ghost!
This short guide explains what means question marks.
Question Marks Explanatory Guide
You can see this ? marks in different places: on the railroad, in the water at the lumber mill etc.
From what I’ve seen those are the locations needed for the photograph challenges you picked up.
For the one in the water use the boat launch nearby and move the boat over to the spot, it will take the photo and remove the floating mark.
You have to get into just the right spot and it will trigger.