It covers the same area as a motorcycle helmet including all of the ears, Its casing is thinner, and nearer to the size of the head, and has lengthwise ridges.
It has a transparent visor to keep sea spray out of the eyes. It has a chinstrap.
Its lining is not padded but inflatable; it has an inflation tube (fastened to the chinstrap) ending in a red nipple (at the bottom of the first image) to inflate the lining by mouth to fit it firmly on the head.
It has a microphone and an intercom lead.