When the sun is in your eyes and you cannot see features on the land itself as they are silhouetted by the sun, it's often hard to sight on anything noticeable when open water swimming.
In this case, you can use the horizon itself, a feature on the horizon, even a small dip or rise, something that is on the horizon.
Or, if your swim is not going to take so long that the sun will move, you can even sight off the sun itself...
In this video we use the sun as a great sighting 'landmark', to take us back to the beach:
Notice we mentioned you could use such a bright and obvious object as the sun when sighting both on top of the water and underwater?
This way you do not need to expend valuable energy looking up all the time as the sun, if it's out and if you have chosen to use it as a sighting mark, it's strong enough to be seen under the water too.
This means you can keep your head down longer, increasing streamlining and stroke efficiency. We've exaggerated the look up a bit here to show how easy it is to use the sun for sighting but hopefully you can also see how strong it is and how it can be seen very easily from under the water.