Tap water in Tunisia is safe to drink but some people don’t like the taste and tend to stick to bottled water.
Take a little bit of care with what you eat, if you want to avoid stomach problems. If you buy any fruit then wash it if you can’t peel it. Ice-cream is tempting but in some places it may have been allowed to melt and then re-frozen, which can make you ill. Shellfish sometimes cause problems, but generally the food is fine unless you choose to eat in very basic places.
If you do get a stomach bug, remember to drink lots of water to replace the fluids you lose. Most tummy bugs only last 24-48 hours, but if yours persists then ask your hotel to recommend a doctor. Tunisia has an excellent health service.
Most people have no worse trouble than trying to cope with the heat. Keep covered up, and slap on the sunscreen regularly. Drink more water than you would normally, to replace the sweat, and try not to drink too much alcohol during the day. If you get a slight headache it might be the start of dehydration, so again drink lots of water to get back to normal.