Why does my one eye water and tear constantly eye constantly tears

Be seen. A corneal abrasion or foreign body are among the two more common causes for what you are describing. You would be wise to be seen for an eye exam.
Tearing. Tearing is most often due to reflex tearing due to ocular irritation. This is often due to allergies, dry eye, blepharitis, rosacea, foreign body, and many other causes. Try artificial tears, warm compresses and ointment at night. If no improvement or worsening, see an eye doctor. Tearing can also be related to blocked tear drainage system.Which needsfurther evaluation as well.
Constant . Constant tearing of one eye suggests a blockage of the tear drainage duct. The tears normally flow through the drainage duct ( near the inner corner of the eyelid ) into the back of the nose. Your ophthalmologist can flush sterile fluid through the duct and possibly relieve the blockage. This is normally a quick and painless office procedure. An examination will determine if this is appropriate for you.