I personally dont like wishing wells, but that's just me. I prefer to buy a gift, showing them that I thought about them and went "all out" in a way, to get them something nice. I can afford to give them cash, but I tend to spend more on an actual gift than if I gave them cash.
So whenever I've been invited to a wedding with a wishing well, I still get them a gift. I included a card with my name and all, nothing anonymous. They were still just as happy and thankful for the gift as they were for anyone else who gave them cash.

Do whatever you feel like, cause either way they will love it.