Try to stay in the shade from 11 to 16 hours of the day. The sun is deceptive, it can "fry" your skin for 15 - 20 min., Even from behind the clouds. Tan gradually, in the morning and in the afternoon. Recommend using different sunblock.
Your safety is guaranteed while swimming on the body of water, marked by buoys. There is always a lifeguard if you swam away from the shore, the flow, imperceptible at low tide can carry you far out to sea. Remember also that the buoys on the sea you can descend ski, surfing, catamaran or boat. No dangerous to humans marine fish, snakes along the coast of Tunisia is not usual.