As always, I'll join in the chorus for Koi in Seal Beach. As far as Newport goes, I can't say that there are great restaurants on the beach or bay. But consider Basilic on Balboa Island -- its a tiny gem and very well though of on this board. You can walk around the Island either before or after dinner. For a combination of great view and great food and money no object, drive south to the Studio at the Montage Hotel. Some of the tables have wonderful ocean views (my wife thought so highly of it that she took me there to give me my wedding ring...)
Awww ...dont' believe The Doubters! There IS good food in downtown!
If you want a fantastic steak, try Silvera's. Clubby, warm, with jazz playing in the background. This place stands up to any of the 'name' steak places like Ruth-Chris or Mortons.
Sushi? Try the new Izakaya, helmed by none other than Chef Abe who is legendary for his inventive sushi. More like a Japanese pub, it's being called a place to eat, drink and have a rousing time with your friends. It's around the corner on Walnut.
Want to go down the coast to Seal Beach? Try Mahe.Located in the historic old Glid'er Inn, their fish is wonderful, and their sushi is fantastic too. If you're in town on a Wednesday, they have awesome all-you-can-eat sushi specials that night, iirc.
Seal Beach - 2nd the recs for Walt's and Koi.
Huntington Beach - It is true that Main St. is not exactly a chowhound's paradise, but there are a few places worth a try if you are in the area, here's a recent post:
Newport Beach - you can find tons of recs on this board. My personal favorites are The Cannery for seafood (harbor view), Taco Rosa for upscale mexican (no view), BlueFin for fusion-sushi (newport coast, oceanview from the parking lot). If you want to eat directly on the beach, there is The Beachcomber.
I agree with "teach" that there's no good food on HB's Main St. Stick to Seal Beach's Main Street. Walt's Wharf and Koi are good choices. Bbq at Beachwood BBQ is quite good, and you can walk down the block to the pier if you need a beach fix. A nice waterfront breakfast is at Schooner or Later on Alamitos Bay (off of Marina Drive, behind the Whole Foods). There's usually a wait, but it's a nice setting overlooking the marina and main channel. There's nothing really special on the beach. Duke's in HB overlooks the pier and the beach, but is just a slightly above average fish house, with a Hawaiian/pan-Asian bent. Other than that and Ruby's (at the end of the HB, SB, and Balboa piers), there's nothing extraordinary beachfront that comes to mind.
You can find great sushi at Koi in Seal Beach. Walt's Wharf has good seafood, but they don't take reservations, so there can be a wait. There is a tiny place right on the beach called River's End (I think) in Seal beach as well. It's kind of a beach shack, but I remember them having a good breakfast.
I would avoid eating in Huntington Beach (except at places like Wahoo's for fish tacos) if I were you. The food is pretty mediocre anywhere on Main St.
I'm must agree that the food options are a bit dismal in HB. When I have friends in from out of town I usually recommend Dukes. The food is average but the view is great (right on the ocean.) On the other side of PCH is Fred's Tacos. It is a younger crowd and has a bar scene. I think the Mexican is decent and you can get a view on the outdoor patio. They have good margaritas.
Up further on Main Street you have Tuna Town, a pretty good sushi place. Also someplace I don't eat at regularly but friends from out of town seem to like it. It is owned by the drummer of the band Korn and has a huge jelly fish tank. For breakfast you should try Sugar Shack if you can. It is one of the few non-chain restaurants on Main Street (and owned by a local family). Unfortunately it is pretty touristy so you need to get there early. A good lunch spot if you do a day at the beach is Jan's Health Bar. It is farther up Main Street and has great sandwiches. The coffee shop inside Huntington Surf and Spot is good.
Also agree you need to stop by the Wahoo's on Main Street.