There are plenty of petite(blue) and grande(yellow) taxis available at the ranks in Essaouira, or even to flag down.  White taxis were not registered as taxis.

When hiring a car from the numerous agents in Essaouira, make sure that you get ALL the required documentation, and there is plenty, keeping it safely. Barter for rental prices as they differ greatly, depending on the type of car hired. A family sized "logan" costs around 25 euros per day, check the car thoroughly for scratches, dents etc. before hiring,and ask if the insurance/breakdown is all inclusive.

Most hire cars have completely empty tanks on collection, fill up before your journey. A word of warning: watch your speed at all times,especially through towns, and the police may stop you on entering/leaving villages and towns. If you are stopped for speeding etc, make sure that you get a valid ticket before parting with your cash to pay the fine.