A boat loan is ordinary amortization:
monthly = principal × r × (1+r)^n / ((1+r)^n − 1)
where r is APR divided by 1200 and n is months. If the rate is zero, it is just principal divided by months.
BoatCost defaults to 8.3% APR, 10 years, 20% down. That is an illustration rate, not a quote. Enter the APR on a written offer.
The gap that surprises people
On a $55,000 bowrider with $11,000 down, the payment is only one line. Florida trailer storage, insurance, registration, fuel, and maintenance often double the all-in monthly number.
The loan does not care whether you launched. Insurance, a wet slip, and winterization do not care either. Fuel and maintenance do.
If a dealer is selling the payment, open the ownership calculator and add the rest of the bill before you compare it to a boat club or a rental.