Osteria Santa Marina is excellent and an easy 10 minute walk. I believe that the 15th falls on a Monday and I believe that many of the great seafood-centric restaurants may be closed. As you are staying at Monaco, you could just literally cross the street (out one door and in to another) and share good food at Harry'sBar. Great ambiance, huge portions and if you order right it won't break the bank. Excellent risotto dishes and they may be one of the only restaurants in Venice which has fresh white truffles. Sharing/splitting dishes is key to ordering at Harry's Bar. It's a fun place which we have enjoyed for many years. Service is great.