That is always a difficult one, because which suit do you need at which water temperature? One dives in a T-shirt in 29 degrees while the other still likes to wear a 5mm suit. It is therefore an individual choice.
And as tempting as it may seem to dive into just a shirt, wearing a long suit that completely covers your body is wise. It prevents you from damaging your skin if you accidentally touch the reef in a strong current or while swimming into a wreck. Or how about jellyfish bits or water fleas? The diving suit protects you against those things, so whatever thickness suit you want, a long suit is always wise.