From Openwaterpediatriathlete. Bubble caps are generally soft with plenty of air pockets molded into the swim cap that provides a snug fit and a chin strap for comfort and warmth. Some swimmers occasionally double cap when swimming with a bubble cap.
Bubble swim caps are made of a very thin latex rubber formed into an air bubble design. They are available with and without chin straps, as well as half cap and full cap design and styles. Manufactured since the 1930's, bubble caps have been worn from swimming pools around the world to swimmers in the English Channel.
Bubble caps are typically excellent for people with lots of hair and can be made of latex or silicone.