There is absolutely nothing wrong with Frozen Fish, as long as it is good quality Frozen Fish and no-one is trying to sell it to you as Fresh Fish!
In the past, practices used by processors gave Frozen Fish a bad reputation. Freezer burn was commonplace, old fish (past any sensible, usable life) was boxed up and frozen down, resulting in a very poor quality product when defrosted.
Modern techniques involve freezing quickly in blast freezers on the boats, within minutes of coming out of the water, and freezing fillets, whole fish and seafood "IQF" (individually quick frozen) with a protective ice glaze to keep the product from drying out. These methods help to preserve the quality and flavour of the fish, and while I will never claim that Frozen can be quite as good as Fresh (a lot try, but I've never been convinced) it's an excellent alternative and is certainly better than most of the "fresh fish" for sale in supermarket counters (in my, ever so humble, opinion...)