Yes, you can pay off your cruise with a travel rewards credit card and earn rewards for that. For example, if you put that $2,000 on the Discover it® Miles, you will earn 3,000 miles ($2,000 * 1.5 miles). And since the first-year miles are matched for new cardmembers, Discover will double all the miles you’ve earned your first year at the end of the year. So basically those 3,000 miles are 6,000 miles. The miles match offer is available to the new cardmembers only, and your miles will be doubled even if you use them up by the end of your first year.
It may take one or two billing periods for your miles to post in to your account. Once you see your miles you can use them – redemptions start at 1 mile. You can redeem miles for travel purchases made within the last 180 days. Travel purchases are airline tickets; hotel rooms; car rentals; cruises; tour operators; vacation packages purchased through airlines, travel agents, online travel sites; local and suburban commuter transportation, including ferries; passenger railways; taxicabs and limousines; and charter/tour bus lines. If you prefer cash, you can redeem miles as electronic deposit into your account.