I shot a birthday party on a boat before and that was a ton of fun! Here is my blog entry from that party with a few photos.
http://www.suechoiphotography.com …salito-event-photographer
I used 5dii + 17-40 and then 50L on my crop for 90+% of the shots. Few fisheye shots were cool. I didn't use my rented 70-200 at all mostly because I didn't need the reach (believe it was a 80' schooner but felt quite small once on it). I found I couldn't really shoot across the boat because of all the poles and sails in the way anyway. Plus I really wanted to minimize lens changes with the boat rocking a good amount. I would recommend 17-35 on your 6D, bring 70-200 in case, and rent mid prime (35mm 50mm or even 85mm) for your 7D. I would suggest flashes on camera for fill. I found a lot of poles, ropes, sails and such move around during the sail, so even if you found a good place to somehow securely mount a flash, it might get blocked or even knocked over. Definitely ask the crew about where you can stand, where you can try to mount things, etc. I had Think Tank Speed Freak on me, double strap with two cameras. No tripods or light stands for sure. Keep things simple & enjoy the ride. Good luck!
-Sue 