Bummer! You'd think they would have one for those of us that won't ever get our passports filled any other way.
You can ask at guest services the night before. They'll usually be able to have the local Bahamian immigration officer stamp it when they clear the ship for docking at Castaway, since it is a Bahamian island. The stamp will either say "Abaco" or "Sandy Point."
We sailed in January and tried to do this. We were told that it couldn't be done.
We did, however, take our passports off the ship in Nassau and got them stamped.
Probably ran into a cranky CM. We've done this in 5 cruises so far. They usually say "oh, we'll try, but no guarantees" and we've always been surprised with the stamps the next morning....
Some day you might even have a chip under your skin as a passport. My dog already has a chip in case he gets lost.
-Paul