Why is Dad making snarky comments about vegetarians? It's easy to head those off... "But Dad, I'm not a vegetarian. I eat fish." It's also fine to tell him that you aren't trying to get attention or create work. Assuming that you only mention that you're a pescetarian when it's relevant (say, when discussing your plans for eating with someone else) and that you're cooking your own food when you eat something different from your family, his point isn't valid and you can call him on it. (Be polite. He's your Dad.)
If you talk about being a pescetarian and all of the health benefits regularly, then it's easy to see why he might think you're being obnoxious. Even if you don't direct it at him, if he's around every time you discuss your reasons for not eating land animals, it's going to be repetitive and sound like you're on a high horse. When people ask you about it, keep your answer succinct. People don't want a long explanation when they ask you why you're a pescetarian. They're just making conversation. Just saying, "Mostly environmental reasons" is plenty. If they want more information, offer to discuss it another time. Say, "Oh, everyone else has heard my spiel about a gazillion times. I'll send you an email about it later."
If you're asking your Mom or Dad to cook you a special meal every other night, then you ARE inconveniencing them. (Stop it.) Start cooking for yourself as much as possible, and make a point of cooking for the entire family once in awhile. Show that you're making an effort to take responsibility for your choice. You don't need to knock yourself out making something elaborate, but consider making four portions rather than one sometimes. "Mom, Dad, I found a great recipe for fisherman's pie that I'd like to try. Can I make it for us on Sunday night?" or "Thanks for getting me the snapper. Can I broil it for everyone tonight? Will you help me make that yummy rice pilaf to go with it, Mom?" If Mom/Dad does make you something special, don't just say that you appreciate it. Show them. "Thanks for making me a shrimp kebab, Dad. Can I help you make the marinade/do the dishes?"
(It's really hard to be mad at someone who just made you an awesome meal and then cleaned up.)