Well, yes, glassy eyes can be a sign of dehydration. Generally speaking, if a person's eyes appear shiny or have a glassy appearance that means he/she is suffering from glassy eyes. And according to some experts, when someone has glassy eyes, his eyes will have a feeling of listlessness, and the eyes are unfocused, and without any luster. And when you have dehydration, it will cause the eyes to become dry, and leading to glassy eyes. Also, some people who are heavy drinkers will have glassy eyes too. So just pay more attention to your eyes.