I belive the way it works is, if you win $600.00 dollars or more they give you a W-2 form. Thus they (casinos, horse track, etc) report the winninigs for you. Any winnings under $600.00 is your responsibility to report to the IRS. Thats why you see people at the race tracks by multiple tickets for the same bet. So if all the tickets win thier under 600.00 and they don't report it.
Not sure how they do it on the ship though. I guess it's just your responsibility.