Beach or Central Ave, St Pete
Assuming by the word "Beach" you mean Beach Drive, and not just the beach area in general (ten miles west of Beach Drive), then you basically are asking about the downtown waterfront. A few months ago, my family had a birthday brunch buffet party at 400 Beach Seafood and Tap House. The food was excellent, and the service was good - especially for a buffet. They readily agreed to use our centerpies for the table dec, and servers were attentive to plate clearing and those clamoring for more Mimosas. They also honored a discount coupon we printed off the internet - for all 18 of our guests!
There are a number of restaurants I'd recomment in that area: Cassis, Bella Brava, Parkshore Grill, 400 Beach, and Marchand's inside the Vinoy. If you have the $$$, I'd splurge on Marchand's - the hotel is beautifully decorated for the holidays and it would be very festive IMO.
Of course, you must call all these restaurants if you plan on eating out Dec 25th, as some independents definitely will be closed that day.