Retrograde Ejaculation: This condition occurs when semen enters the bladder instead of exiting through the penis during ejaculation, often resulting in a sudden change to watery or absent ejaculate. It can be caused by various factors including diabetes, certain medications, or after surgeries like prostate or bladder surgery. Given its direct impact on semen consistency, it's a plausible explanation for sudden watery semen.

Ejaculatory Duct Obstruction: Blockage of the ejaculatory ducts can lead to a reduction in semen volume and potentially result in watery semen. This could be due to infection, inflammation, or congenital conditions.
Prostatitis or Epididymitis: Inflammation of the prostate or epididymis can affect semen quality and consistency, potentially leading to watery semen.
Hormonal Imbalance: Certain hormonal imbalances, particularly those affecting testosterone levels, can impact semen production and quality.

Testicular Cancer: Although rare, testicular cancer can lead to changes in semen consistency among other symptoms. Early detection is crucial for effective treatment.
Spinal Cord Injury: Trauma to the spinal cord can disrupt the normal ejaculatory process, leading to changes in semen consistency or retrograde ejaculation.

Young's Syndrome: A condition characterized by obstructive azoospermia (blockage preventing sperm from mixing with semen), chronic sinopulmonary infections, and normal sweat gland function. It could potentially lead to changes in semen consistency.
Cystic Fibrosis: While primarily known for its respiratory and digestive effects, cystic fibrosis can also lead to congenital absence of the vas deferens, affecting semen quality and consistency.
Medication Side Effects: Certain medications, such as those used to treat depression, high blood pressure, or prostate issues, can have side effects that include changes in semen consistency or ejaculatory function.