I used to do the block of ice thing. That really is the best method. But yeah it takes a lot of room. I never used to pre freeze before vacuume sealing but every spring I would find half of my catch with a broken seal and into the veggie garden it went. What a waste
Ya encased in solid ice is THE best. Ive had 2-3 year old fish that tasted like it was fresh catch. Well, maybe not quite, but it was still fresh and awesome tasting. By contrast I've had some vac seal jobs where the seal broke a bit or a lot and the fish definitely got a bit freezer burn and the taste plummeted noticeably.
A buddy of mine does a different system and he swears by it: He cuts his fillets in portion size chunks, puts them in a ziplock freezer bag skin side down. Then carefully adds just enough water so the fish has a layer of water on top of the flesh. He then lays the bag on a cookie sheet and into the deep freezer. He says the fish will have a good layer of ice on the top/flesh side where you want the extra thickness. The bottom on the bag will have a very thing layer as the skin is pressed against the bag, but pretty unlikely the freezer burn will go through the skin.
Ive honestly had SO many Foodsaver brand bags pop the seal in the freezer (even with par freezing the fish to stiffen it up and seal better) that I'm not going to do just this any more. I like the idea of coating the fish in a layer of ice and then vac sealing it. Going to try this next time.