there are bus and train between Seville and Cadiz

Cadiz is a port on many many cruises.  I think we have been there at least 8 to 10 times.  Too many times that I have lost count.  Will be there again in March! 

Seville to Granada is way too far - you spend TOO MUCH time on the road both ways and only have very little time for what is worthwhile to see at Granada, namely Alhambra. 

I were you I would heed Alex advice, visit Jerez and Cadiz instead.