Can I get a cheaper interest rate if I refinance?
Possibly yes but that is dependent on a number of factors. This will depend on the interest rate on your existing loan compared with the current interest rate. In 2020 interest rates hit historic low levels which would make repayments on a marine loan sourced at those rates less than at the higher rate applicable in previous years.
Another consideration is that in refinancing you will be seeking a used marine loan rather than possibly the new watercraft loan that you originally arranged. Individual lenders may attach different interest rates to some older boats.
The interest rate offered on all marine loans is subject to the credit profile and financial circumstances of individual applicants. If your financial situation and/or credit profile has significantly improved since you arranged your existing  loan, you could be offered a cheaper interest rate in refinancing.