It is difficult to book the shark dive if you are arriving to do the shark dive in Roatan.
The shark dive is booked usually a day in advance only (I think it saves them a complex admin scenario) and this presents the first challenge to getting a booking.
You can organise this through any local dive shop in Roatan, not easy though if you arrive on a cruiseship.
If you have your own equipment then prices are around 100 USD+12% tax. This doesn't include the marine park fee of $5 and transfers from the boat.