Many good points in posts above. However, my comments:
1. Buying and selling yachts is not real estate. Clients with a "real estate comparison" mindset usually have more difficulty in their purchasing and selling.
2. The primary selling price of a boat is based on how badly or how quickly the seller wants/needs to sell it. There are no % rules.
3. Boat owners can be thin-skinned. Offering a price too low only sets the stage for a bad relationship. You want to maintain a respectful relationship with the owner during and after the sale. If s/he feels you have insulted his beloved yacht, it won't work in your best interests short term nor long term.
3. Some lenders might still use BUC, but I haven't heard BUC mentioned in many years. The #3 post from Ted/OC Diver is accurate.
4. The questions to ask the broker, aside from a zillion on the condition, are: How long has she been for sale? Why is she for sale? Have there been any offers? How many clients have seen the boat? Are there any surveys? Do you know what the seller will take?
5. Interview the broker. While the "Listing Broker" has a contract to represent the seller, you are now his client and s/he must represent you too. Do your research on the broker. If you are not certain that the broker will be giving you full disclosure on everything, then consider a buyer's broker ("Selling broker"), but do that before you are vested in the Listing Broker representing the boat. Many brokers, both listing and selling, won't work with you if you have already had direct contact with the listing broker.
6. Regardless of economy, boats do sell quickly. If looking for a "good deal," then consider one that has disadvantages (location, lack of pedigree, condition, age..) or an owner with difficult circumstances.
7. A boat by a popular builder, in a convenient location, priced appropriately, and in very good-excellent condition is not the boat to try to low-ball.