There's probably about $8k gross working at $37k. Your dealer will want to keep
a bunch of that, which is understandable because boats are a low volme
business.

Offer a dirt cheap price, but avoid getting into name calling, threats,
ultimatums, and so forth. Keep this a business transaction, not a contest of
wills. The dealer will likely be more willing to sacrifice profit than to
endure being abused.

(There's a special joy in laying away the insulting, belligerent, loudmouth
jerk who thinks that buying a boat three times in a lifetime prepares him to
negotiate with greater skill than a guy who sells a couple of boats a week.)

Odds are that their first counter offer will *not* be their best price. Come up
from you position just a bit. It's often useful to
"walk" and then wait for the phone to ring.
If there's still a lot of ham on the bone, the
manager will be all over the salesman like flies on a cowpie. "How'd that guy
get out of here without buying? You better get him back in here! You have any
idea how much advertising it takes to generate a single 'up' these days?"

Backfire to the "walk" technique:

Almost every time I ever had a buyer walk and then reappear without any further
price negotiation, I knew I had him at the last price the house had trotted
out. He might come in and make all sorts of noise about a further discount, but
in reality he wouldn't be a "be-back" if he didn't see the value in the last
discussed price. Sure,he was going to try to do better, (wouldn't we all?),
but he wouldn't be wasting time on a repeat visit if he wasn't willing to be
closed.

In the end, the most important thing is to get a boat that you like at a price
you believe to be fair, if not extremely favorable.
There's no contest to see who can screw the dealer down to the cheapest deal,
and when such discussions ever do come up "the first liar doesn't stand a
chance" anyway. :-)