To cover all the variations for poached fish would take several pages.
It may be best to give the basics and let the cook try all the variations and short cuts.
Just remember it can be whole fish or fillets; it can be served hot or cold; the poaching liquid can be water or beer, all the way to champagne or a fine court bouillon.
There are several good hot sauces and many good cold sauces.
Finally, decorating your fish with cherry tomatoes, olives, cucumber slices, lemon slices, fresh parsley and pimentos can be as much fun as eating it.
Then, if you really feel ambitious, mold your cold whole fish in gelatin.
My favorite is a whole poached bass, ling cod or red snapper with a white butter sauce.
It's still my favorite even when I poach fillet of sole.

Make a court bouillon:

• In a 6 - 8 quart enameled or stainless-steel pot or soup kettle, bring all the ingredients to a boil over high heat.
• Partially cover the pot, reduce the heat and simmer for 30 minutes.
• Strain through a large, fine sieve into a fish poacher or a large, deep roasting pan which has a cover, and set aside to cool.

For the poached fish:

• Take the fish and wrap it in a damp cheesecloth leaving 6 inches on each end for "handles" to lift the fish in and out of your pan. (If you have a fish poacher, you may not need the cheesecloth.)
• When the court bouillon is lukewarm, lower your fish into it.
• If you use a roasting pan, tie the ends of the cheesecloth by the handles.
• Be sure the fish is covered by 1 1/2 to 2 inches even if you have to add some water.
• Cover, and bring to a slow simmer.
• Cook at a very low simmer for 15 minutes.
• Then remove the pan from the heat, but leave the fish in 10 or 15 more minutes.
• Gently lift out the fish by the ends of the cheesecloth.
• Put it on a cutting board and carefully skin one side.
• Again, lift the fish gently by the cheesecloth and turn it over onto a heated platter and skin the other side.
• Now you can decorate then cover the fish to keep it warm.
• (Save your court bouillon to use as fish stock. )

Link to the White Butter Sauce.