You and your fiance(e) need to pick the date that works best for you. If someone can't make it, then so be it. That's especially true for a destination wedding ... DWs can be very difficult (and expensive) on the guests, keep in mind. It's absolutely your right to have a DW if you wish, but just don't hold it against anyone who says they won't be coming.
It's not wrong of you to plan your wedding if you know for a fact that MOH cannot make it. There is ALWAYS going to be someone who needs to decline your wedding, no matter when or where you decide to hold it. Again, as long as you don't fault her for it, then you're all set. Maybe you can Skype her in to the ceremony or something.
That being said, if it's absolutely non-negotiable to you that your MOH be there, then you and your fiance(e) should talk to her and see if you can work out a suitable date.
Also keep in mind that cruises are a big commitment to ask of people. You risk people getting seasick, or something like norovirus (cruise ships can be floating petri dishes - read the news). Some people don't want to take a cruise. If a cruise is presenting difficulties for your guests, then maybe you should consider marrying on land and then saving the cruise for your own honeymoon.
Also, look into the logistics of the cruise wedding before you make any commitments or deposits. I worked for a cruise company years ago, and while they offered weddings on board they only conducted the weddings while the ships were docked at the port of origin ... that way, guests who didn't want to take the cruise could stay for the wedding and then debark before the cruise left port. The weddings were not conducted out at sea. If this is the setup for your preferred cruise line then maybe your MOH can make it after all, if your departure port is close to where she lives.