Sea World does not generally have a dining plan. Sometimes, during special events, they will offer a purchase plan where you pay one price and eat at certain restaurants. This is not usually advertised online, only at the park...
My suggestions: Take snacks, esp. if you have little ones. Take water, juice boxes, etc. Mango Joe's, right outside of Shamu Stadium, seems to have the best food we have found. There is a new pizza joint where the BBQ place used to be, but we have not been there...
Their prices really are not that bad. I mean, it's a theme park so you are gonna pay for everything. If you like soda and plan to ever return to the park, I would suggest buying a refillable cup...worth it in the end.
Have fun, enjoy it!
Cheryl