Wetsuits protect you from the elements so your main consideration centers on water conditions. You can find wetsuits that provide the protection you need based on the water temperatures and depths of your typical diving conditions.
The diving conditions you prefer determine the style and thickness of wetsuit you need. Water temperatures and depths are the key considerations.
Water temperatures and depths
Water temperature
The warmer the water, the less protection you need; the colder the water, the more protection you want.
Water Depth
Depth may be even more important than water temperature because neoprene--the most common material used in wetsuits--compresses with depth, losing its insulation value. Typically, most neoprene wetsuits lose half their insulation value at 60 feet.
First, they trap a thin layer of water between your suit and skin to reduce heat loss. That's done by the neoprene, which traps nitrogen bubbles to provide the insulation.
Second, by keeping cold water away from your skin, you receive further insulation
The chart below gives you guidance but you should also consider another factor--protection from the elements. Even a thin suit that covers your arms and legs gives you some protection from sun and scrapes.