Allergic Conjunctivitis: This is the most likely diagnosis due to the common presentation of watery eyes and swelling, often associated with allergies. The symptoms can be acute or chronic and are usually accompanied by itching and redness.

Viral Conjunctivitis: Characterized by redness, swelling, and watery discharge, viral conjunctivitis is highly contagious and can cause significant discomfort. It often starts in one eye and can spread to the other.
Bacterial Conjunctivitis: This condition presents with a thicker, more purulent discharge compared to viral conjunctivitis. It can be caused by various bacteria and may require antibiotic treatment.
Dry Eye Syndrome: Although it might seem counterintuitive, severe dry eye can sometimes cause a watery eye due to the body's attempt to compensate for the lack of tears. It's often accompanied by a sensation of grittiness or burning.

Acute Angle-Closure Glaucoma: A medical emergency that can present with a red, painful, watery eye, along with blurred vision and headache. It requires immediate treatment to prevent permanent vision loss.
Keratitis: Inflammation of the cornea that can be caused by infection, injury, or underlying conditions. It can lead to serious complications, including vision loss, if not properly treated.
Orbital Cellulitis: An infection of the tissues surrounding the eye, which can cause swelling, redness, and pain. It's a serious condition that may require hospitalization and intravenous antibiotics.

Sjögren's Syndrome: An autoimmune disorder that can cause dry eyes and mouth, among other symptoms. While it's less common, it's an important consideration in patients with chronic eye symptoms.
Graft-Versus-Host Disease (GVHD): In patients who have undergone bone marrow or stem cell transplants, GVHD can cause a variety of eye symptoms, including dryness, redness, and swelling.
Ocular Rosacea: A condition associated with rosacea that can cause eye irritation, redness, and swelling, often accompanied by skin symptoms.