In my opinion the Groupon deal might be a little dicey... Correct me where I am wrong though, please.
If you don't go on seafood night, the price of an adult buffet is $30 as opposed to $36, so you're saving $6 less, unless the coupon is for $18 off and not $18 for an adult dinner.
Also, if you're not staying on property from what I understand there is a $12 parking fee. Does the daily parking at WDW ($14 if I am not mistaken) include parking at the Swan?
So, if the answer to the parking question is yes and the groupon simply takes $18 off the check, then it's not too bad of a deal.