If you want really fresh seafood, you need to head down to the beach. From Laguna up to Long Beach, there are a great number of fantastic places to get good seafood. Having said that, Orange County (and all of So Cal really) isn't home to any significant commercial fishing, so a lot of the fresh fish is still brought in from Mexico and Nor Cal. It's fresh, not frozen, but not ultra-local.
I personally like Bluewater Grill in Newport, and The Harbor Grill in Dana Point. Closer to DL, there's The Catch on Katella by Anaheim Stadium. That's only 5 minutes from DL. It can be a zoo on game nights, but off-hours and when the Angels are out of town (or the off-season) it's a nice place to get a bit of decent seafood. Not as good as spots closer to the beach, but it does regularly make some top-10 lists. Last time I looked, they were still closed, however they may be back open by September.
King's Fish House usually has a decent variety of fresh fish too. They are a local chain, but I've had some nice meals there. Also on Katella, in Orange so maybe 10 minutes from DL.