Read this link above to find out what you can and can't bring on board without paying duty.
Owing duty is based on honor. Most cruises announce to go to this or that place at 7 a.m. before disembarking to meet with and pay the duty you owe. If you do have a high value item and knowingly skip out and get caught, you will be opening yourself up for big penalties and years and years of getting inspected every time you travel internationally. Many of the carribean cruises go to countries that do not charge duty. so be honest. Chances are if the duty/IRS tax is so little that its more work to collect it than waive, you'll end up getting it waived.
Joe, post 9/11 crap you mentioned. CBP would impede the flow of travelers if they shook down 3000+ passengers. There are many behind the scenes checking of travelers before you disembark that you may be unaware of. Rest easy they are protecting our homeland.