I tried a google of ferry +ibiza +cadiz and found nothing. The ferries from Cadiz all go to the Canaries, not to the Mediterranean. Also, it seem an unlikely route for a ferry or other public transportation since they’re basically on opposite sides of Spain. I found nothing using a similar search with cruise. A cruise is unlikely to be cheap, anyway.

I suggest looking for an on-line Spanish travel agency or an on-line agency that specializes in Spain.

I don’t know of any boat travel for that route. The cheapest way to go would probably be to fly, and there are several ways to string together an itinerary by air. Spanair has decent prices for a 3 hour flight that goes Ibiza-Madrid-Jerez. Another option is Ibiza-Barcelona, and then Vueling from BCN to Jerez. Or find the cheapest flight to Madrid and then take the train down to Cadiz.