"Marine mortgage" means a mortgage which is registered in accordance with the Merchant Shipping Act 1993 and by virtue of which a boat (but not any share thereof) is made a security for a loan.
So a marine mortgage is secured on the boat that it has financed, in a similar way that a house is.
Bank of Scotland also offer a calculator for
Barclays Marine finance have a handy and quick calculator that enables you to work out the repayments on a marine mortgage that you may be considering to purchase your new boat.