Ensenada is on the Pacific coast of Mexico and is fairly close to San Diego, California. It is a common stop on Mexican cruise routes.
I would not say Ensenada is a great destination. It has some things setup for tourists, like a lot of shops within walking distance of the cruise port, but it is not a place I'd recommend going as a destination on its own. The city is dirty, you will probably be harassed a little bit, and there is not a whole lot to do. The water is not a great quality and it is fairly cold, so there is not good water sports like snorkeling.
The main reason people end up in Ensenada is because cruises need to stop at a foreign port and for short cruises from San Diego or Los Angeles, Ensenada is the best option.