I'll leave out the hitchiking and you should too. Leaves us with :
The personal factors are :
Wich one should you choose for your transfer from Marrakech to Essaouira ? It all depends of very personal factors : The 1st factor you should take into account is your personal pleasure. After all you are vacationning .. Now if you are a party of say 4 and going to golf you dont need to read this further. You most probably dont have budget issues so ... Why suffer ?! ... Book you VIP Taxi with EssaouiraTaxi.com now and enjoy the ride from Marrakech to Essaouira directly from the Menara Marrakech Airport or any other pick up point, hotel, riad in Marrakech !
Getting to the Supratours Bus Terminal from the Menara Marrakech Airport involves that you'll need to wait and catch the next Marrakech city bus that will take you to the Supratours station (Supratour is the road extension of the ONCF national railways). The city bus at the airport is not a direct shuttle. It is a regular city bus with many stops. Cost is about 2 euros. Expect anywhere between 30 minutes (if you are very lucky to catch the bus no waiting and no trafic) to over 1 hour if unlucky. So if you are making plans expect 40 to 45 minutes.
You can also take a petit taxi to the Marrakech Supratours Terminal. Expect that it will take you the same time : Between finding a "petit taxi", negociating the price (yikes!) and getting there you certainly will not have saved either time and certainly not money. They take a maximum of 3 passengers.
The shared Taxi Cabs : You find shared taxi cabs at "la gare routière" wich is also the Marrakech CTM bus terminal. Shared taxis are the alternative to buses who leave at fixed hours and often offer unsufficient service to cater for all. So in those taxis you will share the taxi seats with 6 other passengers and the driver (4 behind and 2 in front + the driver). Ever wanted to feel like a canned sardine from Marrakech to Essaouira 175 km ride ? Hint : The driver leaves when all the seats are paid for. Feel like being squished passenger with stinky armpits left and right in the oldest & often unmaintnained mercedes on the Morrocan roads ? Well theres your chance ! The cost is very similar to buses ride between Marrakech to Essaouira and getting there to this bus terminal is similar ordeal to getting to the Marrakech Supratours Terminal. I hear you "Why did you ever mention it then ?" Well because it is a possibility and many do ask the question on Essaouira Travel Forums.
Car rental : This option is a little bit more touchy. Face value, a small car rental comes at a reasonable price to wich of course you will have to add fuel. It can compare to hiring a taxi from Marrakech to Essaouira. If at 1st it may seem like a good idea to rent a car and keep your independance, think again ! Driving in Morocco is somewhat different then in Europe : There are many police road blocks and if one is not careful and knowledgeable, the displeasure of collecting speeding tickets and other infractions can come and eat your vacation money real fast. Add fuel, trafic, local driving habits, parking, accidents risks, etc and if you intend to go in more remote locations, road conditions in some area of the Province of Essouira could refrain you from reaching your destination. So i would suggest you rent a car only if you are familiar with driving in Morocco. And if you really want to have a drive well .. why not rent a quad and let go the adrenaline on the portion of the beaches and sand dunes reserved for this thrilling activity.