I've been on 2 cruise weddings and 3 other destination weddings (I have expensive friends/family). As far as how far in advance I'd like to know, I think realistically speaking, 18-24 months is enough time. Certainly you can start talking it up now, but if you send me any sort of formal invite/STD, it's going in my "to be dealt with later"pile. I think at the 24 mo mark you can start seriously providing guests details. I would envision it going out as a really fancy newsletter type thing first (maybe a STD magnet stuck in) with all the deets: location, price (and payment plan info!), travel agent contact info, magic words to say to ensure you get a group discount or in the right block of rooms, need for passport, etc etc etc. My father proposed to his new wife on Valentine's day. They decided to get married on a Carnival cruise that summer (thankfully, given Carnival's colorful history, we had an uneventful cruise!). We were all notified that spring informally. Fortunately, b/c I was doing a lot of overtime and travel then I had the money, but my youngest sister did not (dad ended up paying her way if she agreed my half sister could room w/us- I got the raw end of the deal on this onel!) and my other sister had just started with a new company and did not have the ability to take leave. Both of these issues could have been avoided with more planning. As you get closer into the actual date, you can do the fancy invites if you like.
As far as location, totally agree that you should do it where it's perfect for you. You only get to do it once right! However, there is another option. My father got married on the ship in port. We sailed from Miami where the majority of his family lives so it worked out that those who could not go (for whatever reason) were able to attend the ceremony and reception but not the cruise. We showed up to port super early. Those of us sailing got checked in like normal, those not sailing had to fill out a form and fork over a form of ID. Then we all got on the ship. The ceremony was held (they elected to have it in the ship's "study/library" and then the reception was held in one of the lounges. We did pictures, bouquet toss, etc from the main stairway. The guests who weren't sailing walked around the ship with us (just enough to make them jealous that they didn't get to stay) and then right before we pulled out of port, they all disembarked. They got to watch us sail off, which made for nice photos later. Just another thought, since you have friends in Orlando - if they can't cruise, maybe they can still be part of the festivities.
What I wouldn't recommend is what my friend did. On our first day in port (in Bermuda) she had us all loaded up in our dress clothes into vans and driven an hour away to this beach where her ceremony was. After the super short ceremony, we all loaded back in the vans and drove an hour back to the ship for the reception while they took pictures. While the beach ceremony was super pretty (and obviously the bride gets what she wants), it was super inconvenient and wasted the vast majority of our day driving back and forth, waiting for vans, etc (not to mention the windy roads in Bermuda and lack of A/C made for some serious car sickness). If we'd had the reception there on the beach and hung out for a few hours enjoying the amazing beauty of where we were, it would have been awesome. But I mostly remember the van ride. I can't tell you anything about the ceremony itself.
Two more thoughts and then I'm done. 1) if someone travels as a single, make arrangements to pair them up with someone else. That's how I ended up on the second cruise wedding. Although I knew the bride from work, we weren't really "destination wedding" level friends at the time. But her BFF had no one to room with so I went along. Totally worth it and glad I did, but if she'd had another wedding guest to room with, it wouldn't have been an issue. 2) this is my personal opinion and I mean no offense: I think if you're holding a destination wedding, you should not ask for gifts from guests who are going with you. After dropping a thousand or so dollars, you still want that $200 gravy boat? Are you kidding me??!! I have ALWAYS given a gift to the destination couple, but it's been out of my heart, not b/c I got a registry slip in my invite. For the cruise where I was a tag along, the bride initially requested no gifts, than changed her mind and decided she'd like some gifts. I'd have given one anyway, but it seems tacky to me to change her mind. But again, guess it's her day.
OH, one last thought, I mean it this time. On one of the cruises we did get a nice little welcome bag with a letter and a beach towel with the date of the wedding on it. It didn't have the couple's name, just the date. That was a pretty cool and useful gift!!
**Okay, I now realize you just asked about invites, so sorry for all my other ramblings.....
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) if we plan for something, we make it happen. 
