Lots of people are "caught" by this issue
You have a CAT C skippers licence, which allows YOU to go 15nm into the sea (daylight hours only).
Your vessel can only get up to a CAT D certification because it only has 1 motor.
Therefore the limitation is set by your vessel and when you are on it you are only "allowed" up to 10km (5nm) into the sea.
With such a "small" craft 10km IS MORE THAN ENOUGH. 10km is flippen far into the DEEP BLUE SEA on such a "small" vessel.
There is even more to it, are you certified for "surf launching" on your skippers licence ? If launching from a river or the beach you need it (for insurance purposes aswell), and even further, check how far you are allowed from a "safe launching site", I don't mean how deep you can go into the sea, I mean you are not allowed say more than 15nm in ANY direction from a safe launching site ( I can't remember how far so check)