If you definitely want a character dinner, you are limited to:
1900 Park Fare
Akershus
Cinderella's Royal Table
Chef Mickey's
Garden Grill
Crystal Palace
Please add any I am missing.
Most buffets (PF, Akershus, CM and CP) will have at least 2 seafood items on the buffet and several vegetarian options. It is impossible to predict exactly what because items are always changing.
CRT has a plated menu, which includes a crab soup and a salmon dish or vegetarian pasta dish.
GG is family style and does offer fish, but also two or three other meats. I am sure they could "hold the meat" and offer you more fish since it is all you can eat.
If you have your heart set on a huge variety of seafood, maybe you could switch your character meal to a day before or after so you could do a primarily seafood restaurant on Christmas Eve?