There are many table service restaurants in the Boardwalk/Beach Club area that won't be full. Don't worry about not having an ADR, you will eat in a nice sit down restaurant on that night, without an ADR.
If you want casual then try ESPN or Big River Grill for burgers and pub food. The Crews cup is also good but may be crowded.
If you want to just have a pizza on the boardwalk there is Spoodles take out window. You can also order pizza and appetizers at Martha's Vineyard at the Beach Club.
If you want a nice moderately priced restaurant then there is the Yacht Club Galley, never gets too crowded. Spoodles also seems to be relatively easy to get in to .
Over at the Dolphin is Palio, a very good italian restaurant that is also very reasonable. There is never a crowd there.
Higher priced meals are at the Yachtsman Steak House (excellent, I'd call during the day to make an ADR there) and Flying Fish. Both Excellent choices. The Blue Zoo and Shula's steak house are also at the Swan (Dolphin?, I never get those two straight).
In General, don't worry. You won't miss out on anything and you'll have a great time.
By the way, Beaches and Cream is the least desirerable restaurant in the area. It's a good place for an ice cream but don't go there expecting great food. It's really just an ice cream parlor that serves burgers and fries.
PS: After dinner if your all over 21 go to JellyRolls, a great place to spend the night.