In many water sports such as scuba diving, surfing, kayaking, windsurfing, sailing and even fishing, bootees may be worn to keep the feet warm in the same way that a wetsuit would. Insulation is proportional to thickness and thus to how cold the water which the user can tolerate; it may be above or below the standard of 5–6 mm of neoprene. In warmer climates where the thermal qualities of the bootee are not so important, a bootee with a thickness of 2–3.5 mm is common. The leg of the bootee may have a zipper down one side or may be tightened with a velcro strap. Where boots are worn with a wetsuit they are usually tucked under the leg of the suit for streamlining, to help hold the zip closed, and to keep foreign objects out.