your text is a bit confusing :o)

but here goes nothing:

owner= Eigentümer

Front owner - difficult to get an exact definition of this, since, at least in the provided context, it is the same as the owner.

a front man is normally the person who, on the surface, owns/is responsible for something - he is the person 'up front'.
it can also mean that the person is the 'pro forma' owner, that is, the owner on paper, but has little or nothing to do with the thing owned
I consider that this is what is meant here - the person has bought a boat as an investment, but wants nothing to do with the everyday running of it.

in the context you've provided, I'd say it's safe to assume that a front owner is *always* a disponent owner, since, if he has nothing to do with the operations of the boat he will automatically charter it to someone else.

I've no idea if there is a special term in German for this.

as to managing owner - if the owner does *not* charter the boat to a company offering boat tours, but charters directly to the end-client, or if he is actively involved in running the boat itself on a day-to-day basis - as would be a captain-cum-owner that would hire the boat including crew (=himself) - then the owner is also a manager-owner.
if this was a shop, and not a boat, then in German you could say something like: "Inhaber-geführt" to indicate that the person behind the counter actually owns the business, and is not just hired to do the job.

hope this helps