In FAQ page 13
Q: Are Mercenaries considered units?
A: Yes
In Shards Of The Throne rulebook page 12 it says Mercenaries are not considered Ground Forces.
So, does mercenary count as ships then?
mercenary
A unit is generally any plastic piece (e.g. ground force, dreadnought,, fighter, space dock, etc.) The FAQ is clarifying that mercenaries are treated as units for any rule or card that references "units" or a "unit" unless the rule or card makes an exception for mercenaries.
Sort of like how fighters are "ships" and are treated as such unless a rule or card specifically calls them out.
The mercenary is not a ''ship'' in regards of cards and rules that specifies ships?