A 7mm scuba wetsuit is suitable for cold water above 7C or 45F and can be used in even colder waters with additional layers. However remember a wetsuit works by trapping a thin layer of water between your skin and the suit and the first thing that must happen is the water needs to flood the suit which means it starts at the surrounding water temperature. It quickly warms up and the use of layering will mean once your body has heated the water layer you should remain warm throughout the dive. A simple way to boost a suit's warmth is to wear a rash vest underneath or a whole body dive skin which will also make getting it on and off easier. Base the thickness of your suit at the depth you will actually be diving, this can be 5C lower than the surface temperature. At the 7mm range most suits will be what is termed semi dry which means that have special seals around the exit points (wrists, ankles etc) to reduce the flow of water, the idea being to trap the warm water within your suit. These suits will be used in colder climates so be aware of wind chill on the surface especially after a dive if you have a boat ride back or a wait on the shore. Here in the UK most suits will be used with a separate hood, although in other areas the hoods can be attached to the suit but this reduces movement. After diving wash your suit in fresh water and allow to dry hanging on a broad hanger out of direct sunlight. Due to the thickness of the neoprene avoid leaving these suits folded for any length of time.