It isn't like at WDW, but it does happen and it can be a great experience.
1st, the shops on board have Pin Trading books up at the register. You have to ask for them, but the CM will gladly pull out the book if you ask. Normal Disney Pin-trading rules apply to the book, you can take any pin you want as long as you give a pin that the book doesn't yet have.
Second, on many (all?) of the cruises (we were on the Fantasy) there is a Pin Trading with the Officers one night (towards the end of the cruise). The officers, (a couple dozen of them) all decked out in their uniforms, each with a lanyard on, make two lines on either side of the main hall. The guests line up and walk down either line, trading and talking with the officers as they go. The line moves well, and you can stay as long or as little as you like. I encouraged my kids to talk to the officers a little, find out about them. They had a blast. They went down each line several times (after a while the lines get short so they weren't in anyone's way). At the end, every pin they had was a keeper (In their opinion) and they had talked with many of the crew at length. Wonderful memories. And I got a few pictures, though no pins.
Have fun!