You will always need some kind of thermal protection when diving, as you`ll be losing body heat into the water.  Short dives in tropical waters may seem ok, but it is still a fact your losing body heat.  In warm waters just wearing a thermal top may be sufficient, or in colder waters, you can use this to layer your protection for better warmth.  These tops can also be worn if you snorkeling, surfing, kayaking, etc; making them a useful addition to your wardrobe.