Yes, you can use credit cards at the Park entrance. I suggest you look at purchasing a season pass for the Canadian National Parks as you pay it off pretty quickly if staying several nights and you have to have a pass or buy a day permit to go north of Lake Louise on the Icefields Parkway, assuming that is on your schedule.
I don't know about getting cash in Waterton but suspect there must be an ATM in or near the liquor store. You won't find a currency exchange (it is a very small town) but businesses will take your American money albeit sometimes at a poor exchange rate. There is a small grocery store in town but I would bring my basic foods with me if I were you. We use credit cards for most of our purchases in Waterton.
Waterton is a great hiking park with a couple of world class hikes (Crypt lake and the Akamina ridge) and many other really nice trails. If tenting, you are wise to stay in Crandell mtn campground since the townsite campground is very windy.
Emjoy!