There are some countries that fill me with a sense of culinary expectation so much that my mouth is almost watering before I get off the plane. Sadly Morocco is not one of those countries. I don't eat meat and haven't done so for over 20 years and the local approach to that is one of acceptance - "OK, she doesn't want meat, we won't force her" which is a step or two better than you'll find in China where it seems to be "The woman is crazy, let's sneak some dead animals into her food when she's looking the other way". Whilst nobody will treat you like you're crazy for choosing not to eat meat, they won't go out of their way to tempt or delight your taste buds. Instead they just give you exactly the same as everyone else but minus the meat. This mostly means that you'll get a lot of over-cooked potatoes and carrots and so much bland couscous that you'll never want to see the stuff again.
I do eat fish and seafood – but unless you are out at the coast I'd suggest to exercise some caution especially when the weather is hot. Don't go crazy as a bit of canned tuna on your pizza or in your sandwich is unlikely to do you any harm but anything that's not frozen or tinned could give your guts a workout that they might not want.
Meat eaters will most likely find that the Morrocan system of slow cooking in a clay tajine makes delicious and juicy meals out of what look to me like the cheap cuts. Certainly the rest of our party seemed more than happy with the available food. Lamb and chicken are the most typical meats and you'll never see pork and beef is rare.
Every must-do list for Marrakech will tell you to eat kebabs in the Jemaa el Fna square after dark. Most likely they'll also tell you to keep a close hand on your wallet to avoid pickpockets and to steer clear of the snake charmers, monkey handlers and transvestite belly dancers unless you want to be asked rather aggressively for money. I've not eaten in the square since there's not a lot of choice that's not meat-based but mostly because I find the place too crowded and intimidating. I do however love to stop and buy fresh orange juice each time I pass through. The colourful juice sellers stalls are lined up around the edge of the square and 4 dirhams (about 30p) will buy you a glass of the juiciest, sweetest, most tangy 'straight from the orange' juice you'll find anywhere. The same juice in a sit down cafe in central Marrakech will cost around 15-25 dirhams but still be excellent.
Bottled water is easily available and I've not heard of anyone getting fake or refilled water bottles. On the square the juice stalls sell a large bottle for 10 dirhams (about 80 pence) or two small bottles for a similar price. Small shops in the medina will be cheaper and a bottle of water will cost you 10 to 30 dirhams in a restaurant – with the higher prices in the swankier places.
The square and the souks are also filled with stalls selling dried fruits and nuts at good prices. The stall holders will wave apricots or dates at you as you pass and there's no obligation to buy. Nut and sesame seed brittles are often available and are sold by weight and I had an outstanding mille feuille slice off a hand cart for just 2 dirhams which I only bought because I wanted to take a picture of the cakes.
The souks and the central old town district have a lot of small restaurants of the wipe-down table and wipe-down menu type and these offer great value with couscous and tajines from around 30 Dirham (£2.50). You might want to take care to avoid salads or uncooked vegetables but tajines and couscous are generally well cooked. Meat on skewers is cheap and very widely available. If you're sick of the local food which is mostly very bland and tends to be the same whether you go to a cheap dive or a swanky restaurant, you'll also find in Marrakech plenty of pizza and pasta restaurants and in the new town there are lots of very classy European-style restaurants.
If you are staying in a riad or hotel that's not so central you may have less choice open to you locally and in the area around our first riad we were unable to find any restaurants or snack bars at all. You may want to consider eating dinner early before you go back to your accommodation rather than hunting around the labyrinthine streets looking for food later in the evening.
Most riads and all hotels will offer food but with the riads it's best to order earlier in the day if you want to eat that same evening. Many places only have a few rooms and won't have a lot of food in the kitchen if they're not expecting you to eat there. On our first evening we had a pleasant and well cooked meal at our riad but we'd ordered it before we left England.
Breakfasts are large but rather basic. In the three places where we stayed we always received bread, juice and excellent coffee as well as jams and hunny, occasionally a fried egg and often the local thick pancakes or a strange fried pastry whose name I never learned. Cake was also often served for breakfast but don't expect a bit UK or US style hot breakfast. Many riads now offer cookery courses for their guests though it's beyond me why anyone would want to learn to cook such dull food.
Vegetarians should take particular note of the delightful Earth Cafe to the south east of the Jemaa el Fna. It was the first vegetarian/vegan restaurant in the city and I've not heard of any others anywhere else. If you have various food allergies, I'd suggest to take care and consider getting translations done of what your needs are. We travelled around Libya last year with a lady who was a gluten-intollerant vegan and had a laminated card with translations of what she could and couldn't eat.
Food is mostly inexpensive and filling although some of the restaurants in your guidebook may prove to be very expensive. Alcohol is almost always expensive and is not always easy to find. On a previous visit during the holy month of Ramadan, we just decided not to even bother trying to find alcohol although some of the people with us got very good at tracking down international hotel chains where they could buy at vastly inflated prices. If you like to drink spirits, I'd suggest to use your duty free allowance from your home country and fly in with a couple of half litre bottles of your favourite tipple.