Assuming it’s made conventionally with a white sauce and mashed potato topping fish pie is a relatively straightforward dish to match. Almost any smooth dry white wine will do.
I would emphasise dry though - I personally don’t want peach, pineapple or tropical fruit flavours with my fish - not in this traditional dish, at least.
Here are some examples:
* Unoaked or lightly oaked Chardonnay - Chablis being the classic example. A very sound, crowd-pleasing pairing. With a richer, more luxurious pie containing salmon, scallops or added cream or with a pie with a pastry lid you could go for a slightly richer style of white burgundy or other cool climate Chardonnay but don’t overdo the oak
* Chenin Blanc - works much in the same way as Chardonnay. Again a more minerally style works better than a fruity one in my opinion
* Blanc de Blancs champagne or sparkling Chardonnay - same reasoning - with a few bubbles. I’d suggest a non-vintage wine rather than a vintage one. Keep it fresh.
* Soave, Gavi and other smooth dry Italian whites - but not Pinot Grigio which is a bit lightweight for a creamy sauce and mash