I train for both BBing and Outrigging and even putting away a lot of calories i don't get near to 12,000. I would be suprised if swimming burnt that much more than outrigging.
Everyone is different. I know a guy who does some long sessions outrigging and surf skiing and he doesn't really eat much at all.
I also find the more junk food i eat the better i look, as long as i don't eat stupid amounts at once and let it all digest ok before the next meal, then i can eat pretty much anything.
Too much clean food makes you fat IMO. There is only so much rice and tuna i can eat i can't really take that much.