Why is a map location marked with white circle?
Solution 1:
The circle highlight appears once you have been close enough to the place to identify what it is but you haven't yet actually collected all the loot.
The circle disappears and the place appears grey once you have got all the loot.
Solution 2:
That means the objective is somewhere within the circle. Just ride your boat out there and you'll see some chests or barrels that you gotta grab.