There is the Bus of Supratours runs between Marrakech and Essaouira every 2 hours, so you can check out the schedule of the Bus if you will arrive during the time that it departs would be helpful, as you can you check the link to see the time: www.oncf.ma .
So, according to your husband's needs you can hire a private driver to pick you up from the air-port to Essaouira, as you can manage that with your accommodation in Essaouira to give you the pick up from Marrakech in an affordable price, and once you need to go back from Essaouira to Marrakech you can ask your accommodation in Marrakech to do the same thing so to earn time and save your money.
and in Case you did not find a driver, you can read the recommendations here and will find a reliable driver or just google it recommended driver for Morocco, the first one comes highly recommended is the driver Jalil: contact details is viamorocco@gmail.com .
Good Luck.

We rented a car from Thrifty at the Marrakesh airport and it was so easy to drive in Morocco. The road between the airport and Essaouira was fine-this was two years ago at Christmas. I think that you would be happy to have a rental car as it gives you a chance to go buy beer/wine at the underground liguor store;drive a little outside of the city and see the infamous "goats in the argan trees" and the magnificent beach just south of there.Unless you are off roading in the mountains,there are paved interstates all over Morocco.

Your accommodation in Eassaouira can arrange the airport transfer for you if you do not fancy the bus ride,
Jalils email address is ajalil197@yahoo.com not viamoroccoxxx, check out the link below for more reads...
http://www.tripadvisor.com/ShowTopic-g293734-i9196-k3590963-Fabulous_driver_recommendation-Marrakech_Marrakech_Tensift_El_Haouz_Region.html

But you can contact him and see if am i right, we have already contacted him one time before, and he has forwarded us by his new e-mail, which is viamorocco@gmail.com .
i only found him on Google search engine, comes

I remember the road between Marrakesh and Essaouira being very good and that was back in the early 80s. I zipped to Essaouira, solo, on the bus and spent half a day there. Then I went back to Marrakesh.

The only problem was that the camels would walk up to the road and the driver would have to stop. They would even come over to the bus and put their heads in the window. A few times, they decided to stand or sit on the road, so the bus driver would stop and we'd all patiently wait until they decided to move. LOL! I still have the photos of it and laugh whenever I look at them.

Happy Travels!

Oh thats back then, big part of the road now is 2 lanes, in an excellent condition.