In my experience you have it exactly backwards. Actually, its the brokers that are more interested in dropping the prices than the owners. Brokers want quick sales, they realize that we are currently dealing with a buyers market and they don't want to be advertising the same boats month after month since that costs them money
Owners are not trading in the marketplace and so they adopt an attitude that they know what they paid, and their boat is in good shape, so that must be what it is worth now. Of course it was a sellers market when they bought thier boat and, as any good broker now knows. it is a buyers market today.
A good broker is your best tool in getting a good price on a boat. I would find a broker that you trust and work with him in making your case. Discuss it in detail. Show the broker listings that you have followed for years so that he can refer to those specific boats.
In framing your offer it sometimes helps to include a note with your offer that explains your offer in terms of the market place perhaps mentioning that the average time on the market for the brand that you are considering is two years or what ever.
Approaching an owner directly with a low offer is only likely to piss off that owner. I know it did me. I refused to deal with that buyer because his behavior reflected someone who seemed to be trying to pull a fast one and therefore could not be trusted.