Good restaurant in Sousse:
Le Gourmet, almost at the end of the road from the medina to the beach, the main one with the shops, it is set back off the road through an archway. Russian owners, Tunisian chef and waiters, all speak an amount of English and all, including the owners, are very friendly.
I've eaten all over Tunisia, in the desert, in 5* hotels, and in various towns and villages across the country, and the homes of local people, and this food is as close to "Traditional Tunisian" as anywhere else, and at a reasonable price. You'll also find little treats such as "Pancake with Ice Cream"
We ate here in April/May 2009 on our way home from down south.
Sorry, don't frequent tourist resorts, only to break the long journey home.
We found plenty of people on the beach in early May, and the water was very swimable, so I guess that by the end of the May it will be lovely.