It's not just for boys, plenty of girls enjoy it, too:
http://www.wdwinfo.com/just_for_kids/piratecruise.htm
My then five year old granddaughter did this and enjoyed it very much. A few things with this cruise to remember -
Parents are not allowed on the boat
There are at least two Cast Members who will be aboard with the pirates
Get to the dock around a half hour before the start of the cruise since you, the parent/guardian, will need to outfit the pirate with a life vest. Once the signing in process starts, you can get that over with and take photos with your new pirate. And note - the life vests will be checked by a Cast Member before boarding and adjustments will be made if necessary.
Parents then get two hours to do whatever they want
Be at the dock when the boat arrives and have your video cameras or phones ready for the docking. I won't say anything about this at all.
If you have a single child (as we did), the child will be paired up with a similar age child, usually of the same sex and they will be "buddies"
We were given a pager and a receipt. We had to have both in order to "claim" our granddaughter off the boat. No child was let off the boat until a parent/guardian was present for the "claim".
The boat takes off at the exact time so don't be late (see third point above for reasons to be early)
Our granddaughter refused to talk to us about the "goings on" on the boat because (and she giggled immensely), "my buddy and I were told not to tell pirate secrets!"
If the kids don't eat their lunch, they can take it with them.