There are a few methods for thawing frozen fish. 1. The day before you want to use the frozen fish, put it in the refrigerator to thaw overnight. This is the best method. 2. If you don't have time to do it that way, you can thaw it by placing it in a zip-lock bag, in a bowl under a very light stream of running tepid (not warm or hot!) water. Depending on the amount and thickness of the fish, this should only take about 30 minutes. 3. You can also place frozen Mahi Mahi in a zip-lock bag and submerge it in a bowl of cold water for about an hour. Again, timing will depend on the amount and thickness of the fish.