IMO, China + seafood buffets = not a good idea and especially any cold dishes. Sorry you are experiencing why food poisoning from seafood can be one of the worst. Likely you've lost a lot of water and electrolytes--if you (or have a friend or hotel staff) can get to a pharmacy, try getting some of those Oral Rehydration Salts (口服补液盐) to mix with clean water and sip away. Sports drink like gatorade or similar a less ideal 2nd choice. Get too weak and you may need to head for a clinic to get an IV rehydration. Hope you get to feeling better soon.