A wetsuit should feel snug everywhere, with no loose pockets at the lower back, armpits, or behind the knees where water can collect and flush. It will feel tighter than normal clothing on land, which is expected, because the suit works by holding a thin layer of water that your body warms. If you cannot take a full breath or the neck chokes you it is too small, and if you can pinch loose folds of neoprene it is too big.

Most divers wear a swimsuit, a rash guard, or a thin dive-specific base layer under a wetsuit. A slick base layer makes a tight suit much easier to pull on and cuts down chafing at the neck, shoulders, and underarms. Avoid thick cotton clothing, which soaks up water, bunches, and makes you colder rather than warmer.

Work upward in small sections, pulling a little neoprene at a time rather than yanking on one spot, and get the suit fully seated at the ankles and knees before working on the hips and shoulders. A plastic bag over each foot and hand, or wetting the suit first, helps it slide into place. Grip with flat palms rather than fingertips, since fingernails are the most common cause of torn neoprene.

As a general rule, thicker neoprene insulates more, so a higher millimeter rating is warmer in the same conditions. Warmth also depends heavily on fit, suit style, seam construction, whether you add a hood, boots, and gloves, how long the dive is, and your own cold tolerance, so two divers in identical suits can feel very differently. Use thickness as a starting point and refine it against your local conditions and personal experience.

Rinse it inside and out in fresh water after every dive day, and use a wetsuit-specific wash occasionally to control odor. Dry it inside-out first and then flip it, always out of direct sunlight, because UV and heat break down neoprene faster than anything else. Store it hanging on a wide-shouldered hanger or laid flat rather than folded, so you do not set permanent creases.

Lifespan depends far more on use and care than on age, so a suit dived hard in sun and chlorine may soften within a couple of seasons while a well-rinsed occasional-use suit can last many years. Signs a suit is finished include compressed neoprene that no longer feels buoyant or warm, splitting seams, and looseness in areas that used to be snug. Repairing small cuts and seam separations early adds meaningfully to a suit's life.

A full suit covers arms and legs, a shorty stops at the upper arms and thighs, and separates such as tops, bottoms, and hooded vests layer to add warmth where you want it. Layering a vest or top under a full suit adds core warmth and can extend the range of a suit you already own. Choose based on how much coverage you want, both for warmth and for protection against stings and abrasion.