The best way to cook fresh fish is to put it into a plastic bag with as little air as possible (there are vacuum sealers available for that, but manually squeezing air out of the bag should work too), and place it in a water bath.
The plastic bag is necessary to prevent water access to fish while cooking. You need to keep temperature of that water at 132 Fahrenheit (55.5 Celsius), not less, not more (at 145F it'll already be overcooked). Depending on how thick is the piece it might take from ~20 minutes (half-inch thick) to couple hours (~2 inches thick) to make sure the entire piece was brought to this temperature (especially important if starting with not fresh, but frozen fish). After that you could just fry it for 30 seconds on each side to give it familiar "fried" (or use a propane torch from Home Depot) slightly "brown" look.
If you don't want to mess with plastic bags and keeping water temperature precise, still using a digital thermometer with a needle type sensor will help a lot in any cooking. The "doneness" of fish (and meat) depends only on what temperature you brought it during the process.
If currently you're simmering it — temperature is way above 132. It's most likely close to 220. Means — severely overcooked.