Wetsuits work by trapping a thin layer of water next to your skin and come in a range of thicknesses, choose one which is snug but not too tight. 7mm wetsuits are ideal for cold water, modern 5mm wetsuits are ideal for about 22 to 25 water temperature and 3mm wetsuits or less are ideal for the tropics
The first thing you must decide is which thickness of wetsuit is best for you. Generally we would always recommend going thicker rather than thinner if you are unsure. It is unlikely that in the water you will become too hot. In tropical waters consider a full length wetsuit to protect your arms and legs from abrasion and the resulting stings. The time when you are most likely to overheat is waiting to enter the water, so try to leave suiting up until last and drink plenty of water.
Remember if you plan on deeper diving the water temperature will get colder as you get deeper. There may be several layers of water temperatures, so plan for the deepest part of your dive.