The neoprene is 3mm and a Velcro chin strap allowed plenty of room for adjustment, yet stayed in place well as I swam. I wore this cap with a bright normal swim cap on top for visibility ...

Some people love all the neoprene; from gloves and boots to a full swimming wetsuit, and others just like to wear their normal bathers. There's no right or wrong, it's just whatever makes the swim ...