Nice piccies!!
Anything that suits can be used as a tender, and I would describe an inflatable like those as tenders...the cat rating for these is inshore sheltered waters,c, not ocean, whatever such may mean, lol!...as it relates at least in part to stability ratings.
As a tender for example, I use a 3 metre RIB (Rigid hull... not inflatable keel, not that it would matter) with an 8 hp engine, but rated for up to 15hp, as a tender: 8hp will get 2 and gear on the plane on our RIB, and a 25kg engine is as heavy as I would wish to lift over head, on the boat, sad ol' git that I am!!
Get a 15hp on there lol there is only a few kg weight difference (2 stroke)
Government boat categories:
As you say the directive defines the boat, the waters she may sail upon are part of that definition so category C waters are "inshore" and D are "sheltered":
Design categories (as in the amended Directive)
Category A - Ocean: Designed for extended voyages where conditions may exceed wind force 8 (Beaufort scale)
and significant wave heights of 4m and above but excluding abnormal conditions, and vessels largely self-sufficient.
Category B - Offshore: Designed for offshore voyages where conditions up to, and including, wind force 8 and
significant wave heights up to, and including, 4m may be experienced.
Category C - Inshore: Designed for voyages in coastal waters, large bays, estuaries, lakes and rivers where
conditions up to, and including, wind force 6 and significant wave heights up to, and including, 2m may be
experienced.
Category D - Sheltered: Designed for voyages on sheltered coastal waters, small bays, small lakes, rivers and
canals where conditions up to, and including, wind force 4 and significant wave heights up to, and including, 0.3m
may be experienced, with occasional waves of 0.5m maximum height, for example from passing vessels.
Craft in each Category must be designed and constructed to withstand these parameters in respect of stability,
buoyancy, and other relevant essential requirements listed, and to have good handling characteristics.
The inshore term is interesting as The EU , British Govt., Met Office, Defra etc. - all have different definitions - I have come across "inshore coastal" used as a single phrase.
There seems to be a general acceptance that 'inshore' = up to 6 miles and 'coastal' = up to 12 miles.
However some distress flares sold for inshore state they are only visible for up to 3 miles!
All my inflatable boats now and in the past have been cat C. . . . certified for use up to 6 miles off shore gale force 6 and 2metre waves. We don't usually go as far as 6 miles out, usually 1-3 miles is enough for sea fishing.
I would say the the cat D boats are what's known as tenders nowadays.
The cat C inflatable boats are more than capable and safe for ocean use in accordance with the 6 mile (or 3 depending on your flares
) offshore limit and 2 meter wave height of course
You gotta love this one lol cat c boats in action big swells