What are reasons for blood in seaman?

Inflammation: Infection, non-infectious problems including cancer, and other physical abnormalities. This should not be ignored. Please go see a urologist and have this worked out.

Broken vessel: Blood in seamen is often due to drunken brawls. But blood in semen is usually due to a broken blood vessel in the urethra, in which case it's nothing to be concerned about. However, it's not a bad idea to get checked for sexually transmitted diseases when this happens. Older men may need further evaluation to rule out malignancy.