The official answer from
DCL:
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http://disneycruise.disney.go.com/faqs/prohibited-items/toys/
Prohibited Items
Question: What toys are not permitted?
Answer: Toys resembling weapons (guns, knives, swords and other implements) cannot be brought onboard. If a Guest arrives with items that fit this description, standard security procedures surrounding these items may result in disappointment and delays in boarding.
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This is one of those areas which is in the rules but DCL never enforces. I suspect they keep the language in there so they can enforce it at their descretion if there is a realistic look alike. I have never read a report of DCL confiscating toy weapons.
You will see all sorts toy knives, swords, and guns on pirate night. You will see toy guns and swords for sale in the gift shop onboard.