A lot has been posted on this board about Mermaid Inn, but thought I'd add my voice to the fray. I finally got around to eating there; I live around the corner.
We arrived about 8:45 on Monday night, didn't have any trouble getting a table, and sat in the back room. As others have described, it's designed as a New England fish shack, very pleasant atmosphere. The service was attentive & efficient.
We started with the chilled pea soup topped with generous chunks of lobster & drizzled with chili oil. I'm a sucker for cold pea soup, and it was well done, so I really enjoyed this. Then I had the whole grilled dorade. Really tasty -- flesh was firm & moist, and the spicy escarole accompanying it, studded with golden raisins, was an unusual, successful accompaniment. My date got the spicy spaghetti with shrimp, bread crumbs, anchovy & broccoli rabe, which he also really enjoyed. After the meal they bring you a complimentary cup of chocolate pudding.
The reviews on this board have been mixed, but I'm on the positive side of the fence. It's better than your typical neighborhood place, and a good value. FYI the menu posted on menupages is outdated.