Disney prohibits realistic toy weapons. But, I would guess lightsabers would be okay. But, if they are an issue, I think Disney will hold them until you disembark.
We were on a Star Wars cruise on the Fantasy last year. Light sabers were sold on the ship so I think you will be fine.
BTW, there were two very different price points last year. One was over $100 and sold in the shop with Star Wars apparel. In the shop with children's toy, there was another version which was $20 odd IIRC.
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