I think sighting is all about awareness in the water. When driving a car you don't just look straight ahead and line the car up to the left of the white line. You also look at other things to give you an awareness of your position on the road such as road signs, hedges, other cars. You can do the same in the water too and it can help you to avoid collisions with other swimmers
If there are weeds and a current then the weeds will all face in one direction. You can use them in a similar way to lines on the bottom of the pool, although you might want to be swimming at an angle to them rather than lining up with them depending on the direction you want to head in. This is a good trick over short distances and can keep you on line between sighting ahead of you.
Also look at the bank and try to keep in line with that. Make sure you draw an imaginary line along the bank and follow tthat rather than following the actual bends and curves in the bank. Again, this is good for short distances coupled with sighting ahead.
When sighting ahead, try to look for something beyond the buoy. Buoys are hard to see in the water especially if there is any chop or other swimmers around. So instead look for a landmark on the bank such as a tree, building or spire that is higher up and easier to spot. The boxes that contain the rescue rings located around a lake are good as they are bright and easy to see. If you get a chance you can walk around the lake and work out your sighting points before you get in the water. This makes it quicker for you to turn at each buoy.
When sighting, take a quick glance. Lift your head just enough so that your eyes are above the water line so that your position in the water isn't affected too much. If you can't see anything then don't worry about it and try again a couple of strokes later. Adjust in small increments as it is easy to over compensate and then check again before you go too far.
Don't trust the people around you to go in the right direction except during the first 100m or so of a mass start where you are fighting for position and space in the water.