Actually, I've been to ones where you can't get within a boat's length of the coordinates. I've had them on server lines, and when the tip of your boat gets near the line (a little over its length away), you pop to the other side, which depending on the boat can be a jump of more than a degree.
But, you can fish it up from anywhere where there is pre-chest loot (which is at least a 1 degree by 1 degree area, probably closer to 2x2).