Deep sea snorkelling is similar to the original snorkel. However, when the giver of the snorkel licks the guys balls and wanks his cock, s/he takes a deep breath and goes down further. Often including rimming of the receiver's anus. Thus looking like a deep sea snorkeller.
Suzy's boyfriend had an especially fantastic orgasm when she began to snorkel him and then surprised him by taking it further and indulging him in a session of deep sea snorkelling.