I have a friend who lives near the ocean in Masachusetts, and he makes ceviche quite frequently. He varies the ingredients acccording to what is available. Sometimes he uses chopped tomatoes, and often adds chopped fresh ginger as he's very fond of that flavor. Some recipes call for garlic, olive oil, various herbs (oregano, cilantro, basil), olives, capers, even Pace Piquante sauce.
The main premice is very fresh seafood marinated in lime (and sometimes other citrus) juice which "cooks" it.
Here is a basic recipe:
Ceviche
2 lbs mixed seafood including a firm fish, clams, shrimp, scallops, etc.
1 very large onion, thinly sliced
1/4 c chopped parsley
5 ribs thinly sliced celery
Serrano chile peppers (or your favorite) to taste
Salt and pepper to taste
Lime juice to cover
Combine all ingredients and refrigerate two hours.