To book the Grand One, you just have to call Recreation & you can book it. Then call Private Dining to arrange for anything you want to eat on the yacht. You don't have to mention that you are having a wedding, even; this is completely separate from FTW, so you don't book through them. If you choose to, you can tell them that you are getting married through Fairytale Weddings earlier that day, and that you would like this fireworks cruise for you & your DH after the ceremony. They cannot legally refuse to serve you, since you are not violating Disney policy. They might try to, because certain FTW cast members have the idea that an intimate wedding is not as valuable as a custom wedding. What you have to do is just be firm with them. I was very firm with my EM's, and I got every single thing I wanted for my wedding day.
We booked our cruise through the GF private dining manager directly, not through FTW. And yes, you can definitely book it for your wedding day - you don't have to wait until the day after!
We just got back from WDW & saw a bride in wedding gear eating at the California Grill. As long as you were married that day through FTW and aren't in the parks, they allow you to wear your wedding dress. That is and always has been the FTW rule. They cannot stop you from doing something that doesn't violate the rules. You will see brides running all over the GF in wedding gear, eating at the tea room, at Citricos, at V&A's.
I think they made the rule because they just didn't want the FTW intimate coordinators to be bothered with arranging receptions for these brides. We pay less, so we have to do a bit more work. It is definitely worth it, though. You will love the Grand One fireworks cruise!
Cheers!
Heather W