Try and look on the company's website and then call them if no info.
If it's a large boat it will very likely have vegan options.
Also, suggest to email them in advance and ask if they have vegan options or would be willing to make some. Then, if they give a positive response, print and take with you so you can show it it you meet any resistance. Probably not needed, but just incase.
Also take some snacks with you just incase. E.g. nuts are small and filling and not a problem to leave in heat/cold. Worth a back up plan.
Cruises do tend to cater to older folks, and older folks tend to less likely to be vegan.
I have never been on a cruise.
Good luck.