Many who depart SEA check fresh fish from various vendors in, e.g., the Pike Place Market. The vendors will wrap the fish in plastic bags in a styrofoam cooler with gel packs on top, so that the shipment stays cold for 24 hours. All of this is placed in a cardboard "carry-box" that can be checked. UA will accept these boxes on a "limited liability" basis -- that is releasing UA from "fragile" or "perishable" conditions.
I've checked fish about 10 times from SEA with perfect results (now I jinxed it!). But check your bag tags before the fish goes on the conveyor belt. On the last trip back, a few days ago, my fish almost went on someone else's PNR to ORD instead of with me to SFO.