The allowance for most cruise ships is excellent, it is the air carrier you may have to fly with where you need to check and see what they allow you. We flew with BA to Sydney to join a cruise ship and took 3 cases weigh checked before we left home and all were 18/19 kilos each and we had no problems.
The 1st cruise i went on i was over the baggage allowance, this was with thomson, i had to pay £90 excess baggage allownce, i suggest you weigh your cases on the bathroom scales before you go, as they have got tough about weights. i struggle to keep in the limit for a one week holiday, and am going on a 2 week cruise soon, good job its got a washing machine on board! good luck with the packing!!