Ah ha! Matt's time to shine!
Being a now-displaced Rockporter, I'll list off some of my favorites:
Ellen's Harborside -- A very casual place to get good fried food and chowder. Located at the beginning of "T-Wharf", at the end of Broadway. Not sure of their open/close status at this point. $
The Greenery -- Good cuisine, located in the center of town. My grandmother waited/cooked here for a long time. The food is great. Not your normal fried food shack, more gourmet, but reasonable. $$
My Place by the Sea -- A very nice place at the end of Bearskin Neck (sticking out into the ocean). Great food, pretty setting, chicks absolutely dig it. My brother was a cook there for a couple of years. $$$
You can also head over to Gloucester, where the selection of restaurants is much MUCH better, IMHO. Italian, portugese, contemporary, you name it.
One of my favorite places to go is Mc T.'s in downtown Gloucester, as the menu is enormous, and the food great. I also like the Rudder and the Mad Fish over in East Gloucester, both great places for seafood and the setting is much nicer. Plus, the nightlife around there is hopefully still great (it was three weeks ago when I was there).
If you like Italian, head to downtown Gloucester. Valentino's and Espresso have great stuff. And for really fine cuisine of all kinds, check out Passports -- the food and wine are great there.
Being a Friday night, you could then venture over to the Blackburn Tavern in downtown Gloucester for a good dose of live music and local culture -- order up a 16oz. Schaefer from Steve, the bartender, for me
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Lastly, but definitely not least, if you just want a big plate of fried seafood...well, you can go pretty much anywhere, but I would probably head to Amelia's, as the prices are pretty good for the amount of food you get. It's a sub shop, located pretty close to CAD.
As far as "casual" goes, the restaurants on Cape Ann are pretty laid-back. You can wear nice clothes if you want, but jeans and t-shirts are fine anywhere.
Have fun, and let me know if you'd like more info!
Matt "I wish there was at least ONE restaurant in Nahant!!!" Drayer