"저는 채식주의자입니다." in big, bold font and in parentheses under it: "That includes seafood." - but in Korean. Perhaps a cartoon picture of an animal can accompany it or a fish/shellfish by the last line. I'll leave it up to those with artistic skill. ^^
Basically, this is for all those vegetarians I know are in Korea. This is by no way an extension of those hardcore Anti-animal Creulty shirts - I just want something simple (and maybe even cute) so waiters/waitresses can understand and I can eat without having too much of a hard time due to personal beliefs.