Strangely mushy black cod - why?

One of our favorite dishes is black cod done Japanese style - kasuzuke - marinated with sake lees as the base and then broiled to fabulousness. We have fresh cod in these parts and it's a treat we have enjoyed for years (local, but expensive fish). The last time I made it I was pleased to find the Uwajimaya recipe online so I followed it: mostly what I've been doing, but they have you salt the fish, refrigerate overnight, and then wipe it off and marinade the usual 3 days in the fridge. I didn't dry the fish off that well before marinading it, since there is some water in the marinade I thought it wouldn't matter. Surprisingly, and almost tragically, the final product went gooey mushy at the bottom of the fillets - toward the skin side. It's never happened before, and I'd dearly wish it doesn't happen again. Any theories about what went wrong? As you can tell, I suspect the salt/drying process.....