What Is The Weather In April In Hammamet, Tunisia And What Is The Sea Temperature At That Time?

It will almost certainly be dry and pleasantly warm, 21 Deg. C as the daytime high.  But because it's a "dry" heat, that's a comfortable temperature.  Not muggy or sweaty like 21 Deg. C could be in northern Europe.  Most likely you'll want lightweight fabrics, sandals and short sleeves in the day.  Still make sure to pack a light jacket and long trousers as it cools down a lot at night (dry air doesn't hold warmth as well, either).

Sea temperature will be a bit cooler, probably 17-19 deg. C at the warmest.