I once tracked an athlete's GPS across a 1500 metre lake swim. The course was 1500 metres. He swam 1740.
He was not a bad swimmer. He was a bad navigator, and he had spent an extra four and a half minutes in the water for no reason at all, which is more than he had gained from an entire winter of swim training.
Swimming further than the course is the most expensive mistake in triathlon, and almost nobody trains the skill that prevents it.
The Two Ways It Costs You
The obvious one is distance. Sixteen percent extra, in his case, which is enormous.
The less obvious one is that sighting itself is expensive if you do it badly, and most people do.
The instinct is to lift the whole head, look forward like you are treading water, then put the face back down. Every time you do that, the front of your body goes up and your hips go down. You have deleted your streamline for a second and a half, and then you have to rebuild it.
Do it every six strokes for 1500 metres and that is well over a hundred small sinkings. That is the cost of a badly executed sight, and it is often larger than the cost of swimming slightly off line.
Crocodile Eyes
The technique that solves it is a small one and it takes practice.
You lift only enough that your goggles clear the surface and your mouth stays in the water. Eyes just above the waterline, like a crocodile. You are not looking around. You are taking one quick glance.
The critical part is that the sight happens at the front of a stroke, as your hand enters and presses. That press gives you a small amount of lift for free, so you are borrowing from a movement you were making anyway rather than manufacturing lift by heaving your head up.
Then, immediately, your head turns to the side and you take your normal breath. Sight forward, then breathe to the side, in one continuous movement. That way the head is only out of neutral once, not twice.
Almost everybody's first version of this is too slow and too high. It should feel like a flick.
How Often
Less than you think, and it depends on conditions.
In calm water with a clear large buoy, every eight to ten strokes is plenty. In chop, or with a small buoy, or in a crowd, every four to six.
What matters more than frequency is that you sight in a rhythm rather than randomly. Athletes who sight when they suddenly feel uncertain end up sighting constantly and inefficiently. Pick a number, count it, stick to it, and adjust it for the conditions rather than for your anxiety.
What to Aim At
Not usually the buoy. Buoys are low and disappear in chop, and if you are sighting off something you can only see half the time you will sight more often and get less from it.
Find something large and high behind the buoy before you start. A building, a tree line, a crane, a hill. Line it up on the beach during your warm up and use that as your primary reference, checking the buoy occasionally as you get close.
This is why I walk athletes down to the water before a race and make them look at the course from the swimmer's eye level, which is about fifteen centimetres above the surface and looks nothing like the view from the bank.
The Other Half of Swimming Straight
Sighting corrects your line. It does not stop you drifting, and most people drift for a reason they could fix.
Nearly everybody has an asymmetric stroke. If you breathe only to one side, you almost certainly pull harder on the other, and you will curve. Swimming in a pool you never find out, because the lane rope keeps correcting you.
Test it. In open water on a calm day, or in a wide pool, close your eyes for ten strokes and see where you end up. Most people are surprised. If you drift consistently one way, some bilateral breathing in training and some single arm work will do more for your open water line than any amount of sighting practice.
Practise It in the Pool
You do not need a lake. Every fourth length, sight forward three or four times at the far wall using proper crocodile eyes. It is slightly ridiculous in a pool and it builds the exact motor pattern.
Ten of those a session, and by the time you get to open water the movement is already yours.
The Bottom Line
Eyes just clear of the water, mouth staying in, sight on the front of a stroke and turn straight into your breath.
Pick a big high landmark rather than the buoy, sight to a count rather than to your nerves, and find out which way you drift with your eyes closed. Swimming the actual course distance is free speed, and it is available to everybody.