Very slim chance that it might happen, but still you should take a pregnancy test after two weeks.
And next time, plan ahead a bit more to prevent unwanted contact with seamen
have him wear a condom, make sure it is on correctly, and start a regular regiment of hormonal birth control. While Plan B is great for the occasional mess up, it is NOT designed for regular use as a contraceptive. It has a SUPER high dose of hormones that could hurt your body if taken too often. Best talk to your doctor about getting on a monthly BC or something similar.