Pairing wine with seafood isn't a difficult task. Think in terms of weight and substance—delicate raw fish and light, briny shellfish go best with equally delicate, light white wines. Similarly, a piece of grilled swordfish will go better with a richer, more substantial white (and if the fish is served with, say, a red-wine reduction, a red wine may even be the best choice).
Shellfish tend to call for light whites, like Vouvray from France's Loire valley (Benoît Gautier is a good producer), and sparkling wines like Champagne. If the dish is spicy, consider a wine with some sweetness, like an off-dry Alsatian Gewürztraminer (try Trimbach or Domaine Weinbach). If not, consider a Grüner Veltliner from Austria, that country's premier white grape. It produces a palate-cleansing white that, like Sauvignon Blanc, is versatile enough to go with almost anything. The producer Bernard Ott makes good ones, as does Weingut Bründlmayer and Weingut Hirsch.
White-fleshed fish in a butter-based sauce is a good opportunity to drink white Burgundy, made from Chardonnay. There are many great producers; consider wines from Olivier Leflaive, Bernard Morey and Paul Pernot. If the sauce is more citrusy, consider one of Spain's favorite wines for seafood, Albariño, a citrus-zesty white variety from Galicia. Lusco is a good choice, as is Condes de Albarei.
Underappreciated, and thus a bargain in the United States, German Riesling—with its light sweet-ness, green-apple fruit and fresh acidity—is among the greatest of whites. It's delicious with medium-bodied fish such as trout. Moderately sweet Spätlese Rieslings are among the most impressive and versatile; Dönnhoff makes some good ones. Aromatic Pinot Gris from Alsace (Trimbach again) or Oregon (Van Duzer), with flavors that tend to recall tree fruits like pears and peaches, are also good choices.
Oily, darker-fleshed fish, such as mackerel, is poised between white and red wine, depending on how it's cooked: Capers and lemon send it back toward, say, the lively, light-bodied Greek wine Moscophilero (Boutari's is widely available). A mushroom sauce, on the other hand, brings Pinot Noir into play; try a California bottling, which will tend to have cherry and berry notes. Consider one of the many single-vineyard versions made by California's Siduri. Or, stay neutral with a good rosé, such as the rosé of Pinot Noir made by Sonoma County's Balletto.
Salmon also works remarkably well with Pinot Noir, while bright red ahi tuna is so substantial that it can even pair with a medium-bodied red like Merlot (Sebastiani makes a good one).
If you can't decide or everyone at the table is eating something different, order a bottle of nice Champagne—it's one of the most food-friendly of wines. Personally, I like Pierre Gimonnet & Fils, a smaller producer. But the basic brut (dry) Champagnes from the major houses—Veuve Clicquot, Pol Roger, Bollinger, and Taittinger are a few—are some of the most reliable wines in the world. It's hard to go wrong with any of them.