Everyone picks an arbitrary point at which they stop eating animal derived things, on a scale with complete vegans at one end, and cannibals at the other.
99% of people pick a point somewhere away from the extremes, like being a vegetarian, or a meat eater. Unless you're happy to eat dead people, or you abstain from all animal products you aren't at either extreme so you've picked your own brand of 'hypocrisy'.
Being a person who eats fish, but doesn't eat meat, is just another point on the scale. It is annoying though when people like that refer to themselves as 'vegetarians', because it spreads the idea that stupid people sometimes have, that all vegetarians eat fish. Similarly when people who are basically vegan (dairy free etc.) refer to themselves as vegetarian, it can also cause confusion.
There are environmental and historical arguments for not eating meat. Historically (particularly in the 'caveman' days) we ate far less meat than we do now, and our bodies aren't really designed to eat so much meat. Environmentally, meat takes far more energy (something like 10 times as much) to produce the same quantity of food (in terms of calories) than non-meat sources. I have a feeling that non-farmed fish are more environmentally friendly than farmed animals, meaning that this might be a reason someone would choose to eat fish but not meat.