I saw a Rick Steve's recommendation to take a day trip from spain to Morocco.
Rick has shared many, many valuable, useful and wonderful bits of insight and wisdom over the years. He has also said a few stunningly stupid things, too (he's only human, not a god - blasphemy here, I know, but still...). IMHO, his suggestion to ferry across to Morocco for a few hours is one of the worst bits of advice he has ever doled out.
Look: If someone wanted to see Mexico, would you suggest to them that they should go to Tijuana for a couple hours? That's a perfect analogy for taking a ferry to Morocco as a day trip. In both cases, all you get is a grotesque (to use Rick's term), phoney, manufactured-for-tourists experience - the worst possible way to "see" any country.
Morocco is awesome (so is Mexico, for that matter, but a different conversation). Morocco deserves more than a few hours. Best of all, you can fly to Morocco - the real Morocco, the parts you really want to see (Marrakech and/or Fes) for not much more than your day trip ferry nightmare would cost - flights on low cost airlines from Spain and other places around western Europe are dirt cheap, quick, and easy.
I've been to Morocco several times. The hardest thing is getting your head past the idea that visiting it is beyond your skills. It's not. If you can manage Rome or Barcelona on your own, you can "do" Morocco. Morocco is a major tourist destination for people across western Europe - the Mexico analogy works there, too: in so many ways, Morocco is to Europe as Mexico is to the USA: a source of labor (legal and otherwise), culture, food, and more. It's one of Europe's favorite cheap-fun-in-the-sun destinations. Yes, it feels exotic when you arrive (it is!). But it's not difficult, it's as awesome as any place in Europe (the old cities will make you feel like you have stepped out of a time machine), but most of all, it's worth more than a few hours in a "bordertown" (ferry port).
Give Morocco at least a few days, it deserves it (and so do you). Skip the ferry experience, and fly to Marrakech or Fes. You will thank your stars that you did. If you can't carve a few days out of your current trip, save it for next time. But don't take a ferry all day, spending a few hours fending off the hucksters and snake charmers, and come back feeling like you actually experienced the country.
Just my 2 Dirhams. Good luck and As-Salaam-Alaikum.