Your best bet might be to do it the way that folks in the US have done with planes and boats for years. Your tax men
will be looking for an "arm's length" relationship. The further apart you and the "business" are, the more they will accept it as not being some kind of commingling and cheating.
So you set up a business corporation, the corporation owns the boat, the corporation hires a full-time independent manager or other agency to advertise, charter, and maintain the boat. It is a business and you have nothing to do with it, except for being the 100% stockholder and doing the hiring.
When you want to use the boat, you charter it just like anyone else does, at the same price
that anyone else pays. If you have set up a legitimate business and it is making money
on every charter, the money
you've paid comes back to you in profit, so you really are getting a break on the "final" price
. But in the eyes of the tax man, it is still a bonafide arm's length business transaction.
Now, if you also set up some clause allowing corporate officers to bump other charterers on the calendar, or give yourself other privileges...you might want professional advice as to what is acceptable. Or, just verbally tell you charter agent "Before you take any bookings for the month of xxx, confirm the availability with me."
Obviously, it can be simpler to put a yacht in charter with an established company, rather than trying to compete with them. You'd probably find a number of second-tier companies that are willing to take any "reasonable" boat rather than a specific tailor0-made one for their fleet, too.