It makes sense to me. Obviously most of us don't know your local lender or boat markets, but 6% on a 5yr boat loan with 35% for a 11 year old boat seems right. The lender has substantial risk here that is only mitigated by the cash you drop down at closing. Besides, plunking down 35% shows the lender that you might be one of the faithful borrowers.
Maybe you could negotiate the lender to by offering to increase insurance coverage. I'm sure that they have their minimums requirements, but they might lower the down payment if you increase the coverage amounts. I assume the lender will be named on the policy, so you are essentially trading lower up front costs for higher insurance premiums. I'm not sure it's a smart financial move in the long view, but if you're not concerned it may be an avenue worth exploring. Sometimes a straight conversation with the lender can be fruitful. "35% seems a bit much. What could I do on this deal to lower that, or what would a different boat do to lower the down payment?"