Play it safe, declare everything that you have acquired while out of the country. While the Customs Officer has some latitude in making determinations as to what requires duty to be paid and may often suggest ways to reduce the amount of duty you may owe, if he feels that you are trying to deceive; well "Sometimes you eat the bear . . ."
You need to ask yourself: Is trying to get that bottle of Rum in without paying the required duty worth the possible fines, jail time, or having your boat seized?
Here is a quick list of things that are required to be declared. Just remember, not everything that is declared is subject to duty.
If you acquired items in the U.S. Virgin Islands, American Samoa, Guam, or in a Caribbean Basin Economic Recovery Act country and asked the merchant to send them to you, you must still declare them when you go through customs. This differs from the usual procedure for mailed items.
You must state on the CBP declaration, in U.S. currency, what you actually paid for each item. The price must include all taxes. If you don't know for sure, estimate. If you did not buy the item yourself - for example, if it is a gift, you must estimate its fair retail value in the country where you received it.
Remember: Even if you used the item you bought on your trip, it's still dutiable. You must declare the item at the price you paid or if it was a gift, at its fair market value.