My EX once over-cooked a whole salmon on the grill. I spread mayonnaise on it and wrapped it in foil for about 30 minutes. Wiped/scraped mayo off, did my usual lemon-type seasoning, and the fish regained its moisture content and was wonderful!
So I can only assume that it would keep fish nice and moist.
"Count yourself...you ain't so many" - quote from Buck's Daddy