"As I remember MOB (COB as is used today) is considered a MAYDAY, "
Not necessarily. A MAYDAY situation is any situation where there is an immediate threat of loss of property or life.
A MOB situation in 40F water would be a MAYDAY.
A MOB in 77F water at noon with flat water, would not.
In the latter situation, if there was no local traffic I'd probably not even use the radio. If there were other vessels in site, I'd probably call SECURITY to advise them why my vessel might be moving erratically and to let them know to stay clear of the MOB during recovery. In a channel or other area with heavy traffic and the chance someone would hit the MOB, I'd call MAYDAY, because that person was in danger of being run down by other vessels.
It's sometimes a fine line, left to discretion. The question is, are lives and property in IMMEDIATE danger of being lost? If not, you've only got a "damned inconvenience", not an emergency.