Great first date idea – not necessarily because she has to like fishing, or be good at it or anything. More because a night out catting inevitably involves some slow time, when you can sit and ponder together, get some of the serious issues talked about, or just figure out if you can stand hanging out together without other distractions.
Just don’t take her out into mosquito madness on the first date and judge her harshly if she doesn’t like it.
Honestly, there are WAY more important things than if she likes fishing. The qualities that make a good fishing partner are not necessarily the qualities that make a good wife. And if you hope ever to marry and spend serious time with the person, you may want to reserve fishing as “your own” time – married people spend a LOT of hours together.
My lady likes to fish a little. She gets wound up for it about once or twice a year or so. Naturally, I fish a hundred or so more times than that, and for the most part, we’re both happy that way (I do a lot compensatory time in the winter).