A Master Mariner is the professional qualification required for someone to serve as the Captain of a commercial vessel of any size, of any type, operating anywhere in the world. edit

Fishing Vessels are commercial vessels, while in the UK the MCA does not allow a STCW RegII/2 Master unlimited to sail as Master onboard a fishing vessel, this fact renders this opening statement factually incorrect.

Also, historically, a Master Mariner is a person who has achieved both a Master Oceans Unlimited Tonnage as a deck officer AND an Unlimited Horsepower Chief Engineer license. Very few individuals have achieved both levels of Mastery in shipping. This is why they are considered true Master Mariners.