@ Charlie D,
Off the top of my thinking Cap, there are thousands of sauces that would be lovely and sublime for fish, however, it depends on the type of fish too ..
1) Mornay ( Butter Sauce )
2) White Wine
3) Apple Cider (Sparkling) or a Cava ..
4) With Trout: Alsacian Choucroute with saukerkraut ..
5) Salmon: Perhaps a Citrus & Balsamic Vinegar ..
6) Salmon with a Teriyaki ..
7 ) Cod or Grouper ( Thick Firm ) Chunks: you may wish to dredge in almond, hazelnut or pistachio crumbs and sauté in some Extra virgin olive oil .. ( A dish in Napoli, called Frecole ).
8 ) Another wonderful dish is a fish stew: 3 Different types of fish, tomato, White wine, fresh parsley, fish stock / broth, garlic, extra virgin olive oil
(Called: Cacciucco) ..
I have the Paul Bocuse Institute Book on the classic Mother Sauces ..
Le Cordón Bleu book series are wonderful too there is one on sauces ..
Hope this has helped ..
Have a wonderful day ..