Marinades are great for many things: it keeps meat from drying out, prevents sticking during cooking, and infuses food with great flavor. One very important rule about marinating fish and seafood is that a highly acidic marinade, one containing vinegar or citrus juices, can actually cook the fish or seafood. It’s best to use mild marinades for fish and let it sit for short periods of time (2 hours at most). Flaky fish fillet can be marinated for 30 minutes; firm fish fillet for an hour; while firm fish steak can be marinated for up to 2 hours.