Do you mean a bubble that is constantly there or watery eye that occasionally has bubbles then drips? It is hard to answer the question without more specifics - It could be a sign of illness or it could mean the tortoise has an irritation (from sand or something) or simply just dunked its head in water. It doesn't sound "normal" to me to have a bubble in the eye unless it is from a dunk in water and caused by blinking that then goes away. Do you have a tortoise that is having this happen?