There are so many variables in this, that it's really impossible to give you a costing.
Because:
What kind of boat?
Are you buying it outright, getting a loan from a financial institution, getting partners, what?
Where will it be located?
What are the licencing requirements? (Inspection fees, licencing fees, operating considerations for non-residents, etc.)
Where will you running your tours?
What kind of tours?
When will you be running your tours? How often?
What equipment will you need?
General maintenance costs?
Dockage costs in home port?
Costs of fuel in the area?
Costs of provisioning including water for those without watermakers?
Insurance (property & liability)?
How are you handling advertising & booking?
Are you crewing it yourself, or do you have to hire crew?
And more than I can think of right now!
Remember this addage:
How do you make a small fortune in the dive charter biz? Start with a large one.