Good dinner place walking distance from Marriot Harbor Beach
The Marriott Harbor Beach is a beautiful spot. If you walk north on AIA along the beach, you'll come to Cabana Bahia (as mentioned in the other post). It's very casual and fun, inexpensive, people dock their boats there and come in to eat and drink (lots). Further up but still walking distance just north of Las Olas is Cafe del Mar...food is nice, kind of busy and loud on the weekend with outdoor guitar player and belly dancer (a la Ocean Drive in South Beach)...they're also open for breakfast (hard to find). If you keep walking a few blocks north (yes, avoid Beach Place) you'll find one of my favorites...Casablanca Cafe (great atmosphere and acceptable food, in an old mansion, live music on weekend evenings, across the street from the beach, somewhat upscale but still casual, dead by midnight). Then a little bit further up is Shula's on the Beach...upscale steakhouse, view of the beach. Enjoy your stay!!
I know this is NOT what you asked in your original post, BUT for my 2 cents and having lived in and around Ft Laud my whole life, there are way better restaurants than those on the beach. Weird, I know. Definitely some good options on the beach, but if you can get a cab or a car, there are much better places that are driving distance.
For beach recs, lots of people recommend 3030 Ocean, which is at the Harbour Beach Marriott.