Do not put water in the bag to defrost the frozen cut up pieces of whatever. When I go to defrost the food I'm about to feed him, I remove the bag from the freezer, place it in room temp water with the zipper closed, and out of the water. You don't want to be re-freezing your fresh shrimp or whatever in water.
As for choosing food. I go with whatever I can get that's the most fresh. Shrimp, squid, clams. I try to get clams on the half shell, but if all they have are closed shell clams, I'll go ahead and get those as well and just crack em open myself.
Also; I'm completely new to this as well. I have however, literally looked at and done research on GDM for approx 5 months before getting mine. I've read alot on food, habits, methods of keep, etc. Alot of my info came from all this research and listening to the advice I was given, which is what I'm sharing. I am in no way, an eel expert. My eel though, obviously loves what I feed him and shows clear signs of good health. So I'm passing on what I've learned and know!