Can a hernia cause a hydrocele?

No: They are two different conditions although you may have both.

Sort of: These two entities are related, especially in pediatric cases. Incomplete closure of the tunica vaginalis as the testis descends down to the scrotum can be the cause for both hydrocele and hernia. One does not cause the other. That said, hydroceles have been known to occur after hernia repair. This is felt to be related to restriction of the cord lymphatics after herniorrhaphy.