they really should have had a vegetarian option, most do these days - but seafood and fish is NOT vegetarian.
i once went for a set meal with my family and we had to agree to the set menu in advance - when i asked what the veggie option was, they weren't sure, but assured me it would be veggie
i was served a monstrosity of seafood - i was furious.
i am always offered fish when i say I am veggie - it is very irritating.
vegetarian means - no living creature to be eaten - that includes, cockles, mussels etc
i have even seen menus with little V's beside all the fish dishes!
i think restaurants should make an effort to make sure all staff learn properly what these terms actually mean, that includes gluten intolerances, lactose, and generally any kind of food preference or intolerance.
even just some sort of wall chart for them to refer to would be better.
it would also be an idea for diners to be clear and understand too - claiming to be vegetarian and then ordering fish is confusing.