Alright Georgiagator, I played with some math this morning. Something is very wrong with either your math or the dealers math. Lets use your numbers that you have told us.
Boat - 25,000
Payments increased 12%/month and about an extra $4000 total interest.
Interest rate gone up over a full percent
Here are some numbers I found this morning using just a simple loan calculator
Loan for 25,000
5 year at 5%=472, at 6%=483, it would take increasing loan rate to 10% to get to $530 and close to 12% increase. That is plus 5% rate.
8 year at 5%=316, at 6%=329, 9%=+12%. Again plus 4-5%
4 year at 5%=576, 6%=587, 11%=645 which is 12% increase - plus 6% points
So its going to take on average about +5% points to hit your 12% monthly payment increase - not 1%
$4000 interest over 5 years = about $66/month. That isn't doable by an increase of 1% on 25k. On average, 1% increase on rate on 25,000 for 8 years = $1250 and 5 years = $660. Now to match the numbers on the test samples above. A 5% increase on the loan rate on 25k would add about $65-85 month which would add approx. $4,000 over the term. Somebody has a bad number elsewhere. He cut your term length which would increase payment but not overall interest. or he is using the wrong loan amount, etc. It could be a simple honest mistake. But an increase of 5% is not a simple mistake. I would ask to see the numbers - loan rate, loan amount, loan payback and make sure they match up. When I buy a car, I have a loan calculator pulled up on my phone to confirm their numbers are accurate and no games are being played. I double check while I am in the office.