Welcome to cruise-chat. Drinks you purchase in the bars onboard ship will be charged to a shipboard account that is established when you board. You'll be issued a credit-card sized card that serves as your cabin key as well as your charge card for onboard purchases. When you order a drink, the bartender will take your card, and swipe it for the amount of the drink plus a 15% gratuity. You'll receive a bill at the end of the cruise for all of your purchases (bars, photos from the photo shop, gift shop, etc.). So, no need to pay cash each time... actually, it won't even be accepted.
As for shore excursions, you pay for each excursion when you book it... so, if you pre-reserve now, you'll have to have your credit card handy. (This is a very good question as the procedure can vary from one cruise line to another.)