Depending on how much time remains between now and your expected cycle, there may be a possibility of hormonal manipulation of your cycle. Consult your physician, the sooner the better, for your options.
When options are limited, a natural body of water, such as an ocean, may be acceptable to use for immersion. This may be possible to accomplish if your ship docks. See our article on the topic here.
We hope that one of these options can help! Otherwise, you may have to delay immersion until after the cruise. You should be aware that taking a pleasure trip with one’s husband while in niddah raises halachic problems. Therefore, in this situation, the two of you should take special care to be scrupulous with all of the harchakot (restrictions on conduct while in niddah).