Mauritania is still mainly a cash only country.
You did not see any ATM’s. For cash and TC’s you get the best rates in Nouakchott. In Nouadhibou it is quite difficult to change travelers cheques; they try to get up to 15% commission, so you need to bargain a bit on that or change your TC’s in the capital. Except in a few serious exchange bureau’s (not the ones in the market!) in Nouakchott, bargaining on rates seems to be necessary.
High season is from October to March;
At that time about 20 Western overland-jeeps a day cross into Mauritania from Morocco plus there are a number of charter flight arrivals. During the summer months there are almost no tourists (less than 1 overland car a day crosses the border). Even in touristy Chinguetti you don’t see any other tourists. We had to admit that, unless you stay the whole time in an air-conditioned overland vehicle, the temperatures of 45-50 degrees (day and night) combined with the hot wind in the inner country are not made for average humans like us. A camel ride seemed simply not possible in this heat, even the Mauritanian organizers proposed a siesta between 10am and 5pm. On the coast, fortunately, the climate is a lot better. In Nouakchott and Nouadhibou it was very windy and around 25 degrees when we were there in June. This makes the beach-route very possible in summer.
Tap water is very drinkable, especially in the desert where it comes from springs. Unfortunately the water that runs out of the tap in the desert is very often 50 degrees, so you have to put it in the freezer of the guest house to make it drinkable.
It is very easy to find camel meat in Mauritania, and in a lot of places in the desert, camel meat with onions and couscous or rice will be about the only thing available. Try to buy all the vegetables you can find at the market to add some vitamins to your meal, or take vitamin tablets. Outside the main cities, restaurants are very rare so you will mostly end up eating in your guest house. Ask them to prepare camel steak and to use as many different vegetables as possible. If you intend to stay for a longer time in the country (say 1 month), it could be interesting to take a camp cooking stove with you. The local food gets very boring and unhealthy after a while and it is easy to cook outside in most guest houses.