How do I choose frozen fish so that it doesn't release so much water? I bought a package of frozen, individually wrapped, vacuum-sealed pacific cod fillets (about 150-200 grams each) from what I thought was a reputable brand (and not the lowest price) in my nearest supermarket, and attempted to pan-fry it. I defrosted it by running cold water in the sink as advised in this answer (it took about 15 minutes). When I opened the package, a large amount of water had separated from the fish. I carefully dried the fillet with towel paper, coated it with flour and put it in a hot pan over maximal heat as explained in this other question. Despite my efforts and the hot pan, the fish immediately started releasing even more water, dissolving the flour and turning into a small 75-100 grams of boiled, chewy fish fillet instead of a nice fried, tender one.

So I am looking for advice on how to properly choose frozen fish in the first place. I know that it should be properly vacuum-sealed and from a reputable source (whatever that means). But even then, some frozen fishes will loose a lot of water (it was obviously the case here). What qualities should I look for when buying it? Can I tell by the fish aspect or something else if it's good or not? Are some species better than others?
As far as I am aware, you cannot recognize this in advance. 

What you describe is due to very damaged cell structure in the fish. The "water" are the fluids contained in and around the fish cells, which make the filets juicy. They flow out when the cell walls in the fish rupture. 

The reason for rupturing is that the fluids are water-based, and water expands in the 0 to -4 Celsius range. When meat or fish is flash-frozen, it goes very quickly to under -4, and in the small time it spends in the problem range, the cell walls withstand the pressure from the expanding ice crystals. When it is frozen in a "slow" process, the cell walls rupture. Or, if it was flash-frozen, but sometime during storage it spend long periods above -4, it will also have this problem. 

There is no way for you to recognize whether a piece of frozen fish or meat in the supermarket was flash-frozen or not. So sorry, but you have to rely on luck, and maybe try to find if there is a correlation between certain brands and quality by buying them repeatedly.