What causes steamed fish to become mushy? I went to a Chinese seafood restaurant for dinner and ordered steamed grouper. I repeatedly confirmed that the fish must be live "swimming fish".

The fish came out and the flesh was mushy.

Guests who are experienced (self-proclaimed) insisted the fish was dead for quite some time already, hence the protein became mushy, instead of having a succulent texture. However, the manager insisted the fish was live the whole day, until they cooked it for us.

If the manager was not lying, what could have caused the mushy texture?
Assuming that the fish is fresh to begin with...the answer is quite simple, overcooking! Fish cooks far faster than beef, chicken or pork. A great way to steam your fish is to wrap it in parchment paper 
cut and paste the following web address to see how EASY it is to get perfectly steamed fish every time. 
https://www.youtube.com/watch?v=52wYn99cL2I