In the end, I have settled for a very simple and efficient way without any worries about fish swimming in a defrosting water, or actually cooking them in microwave instead of defrosting them.
So I take my pack of frozen fish and drop them into a strainer. Yes, plain old strainer you would use for your spaghetti or pasta.
If I'm going to leave fish over night to thaw, I place strainer into a casserole slightly larger so the bottom of strainer is not sitting all the way down. Cover with lid and place it on the bottom shelf of a fridge. This way all the defrosting water goes down to the casserole and does not stay on a fish.