Actually, it happens more frequently then that. People do this to celebrate their birthdays, or other people's birthdays. Or in mourning, or on a major holiday, or for any reason they can think of.
So when the ship pulled out in September taking half of the winter crew with them, people did what they do.
Keep in mind that this water is about 29 degrees, as salt water has a lower freezing point then fresh water.
One of the things you have to be careful of when jumping after the ship is to wait until it moves away enough from the pier that you don't get caught in the prop wash. If you do get caught in the wash, you get pushed way up the inlet, and face a long swim back to the ladder. Even without the prop wash, it's still not totally hassle-free. You can actually see Waslo getting pulled under the pier bumper by the surge, and had to grab Wally's arm to stop from being sucked under the bumper.