How often have you seen divers in “warm” water with only a bathing suit and BCD on?  I have see it most often on of my southern Caribbean trips.  But beware, if you are diving in 85-degree water (which most of us consider pretty warm), it is still about 13 degrees cooler than your body temp, and as you go deeper it gets a little colder. 

As soon as you enter the water, the cooler water starts to pull the heat away from your body to try and get warmer.  We often see students in our dive pool (which is kept at about 83-84 degrees) start to shiver after 60 minutes in the pool.  So what can you do?
The use of some kind an exposure suit will help keep you warmer depending on the temp of the water.  These suits will also help keep protect you from marine life which can cause irritation such as fire coral. 
There are a variety of exposure suits, which are dependent on the temperature of the water. 

• Skins are very lightweight for warm water above 80 degrees
• 2-3mm wetsuits are useful from about 72-80 degrees
• 5mm wetsuits in 65-72 degree water
• 7mm wetsuit in 55-65 degree water
• Dry Suit under 55 degrees

These recommendations may vary with the diver and body mass.  Also as you add more neoprene it will make you more negatively buoyant.  So don’t be cold get the right suit for the temperature of water you diving.