I would definately suggest heading to Main Street in Huntington then because they have so much to offer and will be able to see some local nightlife mixed in. In Newport there are still great places but they are a little more spread out. If you were set on Mexican the Avila's El Ranchito in Huntington on Main is really great and open!
A little more info would probably get you some better responses. Price point? Limited to only Huntington and Newport, or willing to travel? etc.
Upscale Mexican: Taco Rosa in Newport; Sol Cocina in Newport (haven't tried it personally but seen positive reviews)
For more reasonable and a little less fancy mexican food, head up the hill to Costa Mesa, and try almost any of the places on 19th Street : Taco Mesa (same owners as Taco Rosa), Jugos Acapulco, Taqueria El Granjenal, etc.
For Seafood, you could go fancy and expensive down in Newport Coast at Mastro's Ocean Club.
Or you could combine your Mexican and seafood preferences and head north on the 55 for 15 minutes to Orange and check out Mariscos Puerto Esperanza; their Pescado Zarandeado was the best cooked fish dish I had all year.
Or you could try Wahoo's Fish Tacos, a chain that started up locally and serves up some good grilled fish tacos and throws in an asian twist here and there.
If you are looking for the best Orange County has to offer check out this recent poll of CH favorites:
re: I got nothin
Sol Cocina is actually tasty but the portions are stupid small and not at all inexpensive. A 'street taco' is 2 bites at best, the smallest I have ever seen.
As for the OP, Huntington to Newport is a large stretch of the Coast with a huge range in style and price. HB has lots of smaller places, especially on Main Street. Once you get into Newport, prices and style tend to go up. All the big seafood chains are in Newport on PCH. Oysters is good at the far south end of Newport before you hit Corona Del Mar. If you are willing to go further south, Javier's in Newport Coast is good, with both seafood and nuevo-Mexican, a more upscale Taco Rosa, though I like both. Even further south is Taco Loco in Laguna Beach, better than Wahoo's in my opinion.
There are so many answers to a quex like this, you're probably better off doing some searches on this board.
That said, two quite different seafood places a block apart in Newport Beach (near the Newport pier) are 21 Oceanfront and the Crab Cooker. If you don't like the looks of one, walk over to the other!