All the cruise lines are trying to reel in new passengers these days—some with discounts, others with value-added perks. Taking a page from Norwegian Cruise Line's playbook, today Carnival unveiled its own cash back promo that's good on most three- to eight-day summer sailings to the Caribbean and Mexico from 14 North American ports. Passengers can receive up to $240 in onboard credit per cabin, which can be used to purchase spa treatments, shore excursions, gift shop souvenirs, and other amenities and services. The deal works on a sliding scale: Book a three- or four-night cruise and you'll get up to $100 worth of shipboard credit per cabin. Book a five-night cruise and you'll get up to $150 of credit per cabin. If you book a cruise of six or more nights, you'll receive up to $240 in credit per cabin.

To take advantage of Carnival's offer, you must book by May 10 and sail between May 24 and September 4 in cabin category 4A to 11. Request promo code OCB. 

Note: Norwegian Cruise Line's ongoing offer promises up to $250 in onboard credit, plus a kid's fare starting at $99. As with Carnival's offer, the amount of the credit works on a sliding scale, so price out different sailings in various cabin categories to find the best deal. Use promo code PGWAVE2.