Yes, dehydration will cause flashing lights in front of the eyes. If your corneal is so dry, you will easily see things in blurry state which will make your eyes feel not that comfortable. You could eat more food with vitamin C to moisture the eyes. You could also use the warm compress to make the eyes feel comfortable.
Well, maybe dehydration cannot lead to flashing lights in your eyes. For the causes which can lead to flashing lights, there are some ones you need to know. For example, severe recurrent headaches caused by blood vessel spasms in the brain will last from a few hours to several days. In that way, it will lead to flashing light symptoms. By the way, Retinal detachment or retinal tears can also be a cause. But there is no evidence which has shown that dehydration can lead to flashing lights in your eyes, so you do not need to worry about it. On the other hand, flashing lights can be an indicator of a serious eye problem that needs immediate treatment. Or it will develop into some serious vision problems. So if you have this symptom for a long time, I will suggest you go and see a doctor.