Want to vacation on a private Caribbean island? Just go on a cruise.

• You can vacation on a private Caribbean island for as cheap as $80 a day, so long as you go by cruise.
• Cruise lines like Royal Caribbean, Norwegian, and Carnival have islands exclusive to guests.
• These companies have been racing to expand their private island and resort portfolio.

Winter blues got you daydreaming about a private Caribbean island vacation? As expensive as they might sound, you don't have to break the bank to go to one.

Some are as cheap as $80 a day. Although, of course, there's a catch.

You'll have to go with thousands of other travelers — on a cruise ship, no less.

Many of the largest cruise companies own and operate private Caribbean islands just a day's sail from Florida's main terminals. These islands are only accessible by cruiseliners and are exclusive to their guests.

Once there, travelers can sunbathe on soft-sand beaches, snorkel with turtles in clear blue waters, and, of course, swim with pigs. Maybe for these reasons, travelers seem to love them — especially Royal Caribbean Group's Perfect Day at CocoCay, the company's highest-rated port, Jason Liberty, its CEO, told investors in October.

Families visiting CocoCay can spend their afternoon at the 14-slide waterpark. About a mile away, at Norwegian Cruise Line Holdings' larger 270-acre Great Stirrup Cay, adrenaline junkies can fly through the island's six-part zipline course.

At Carnival Corp's whopping 2,400-acre RelaxAway, Half Moon Cay, guests can explore the beach by horseback.

Cruise ships generally stay docked at these private ports for less than a day. Yet, the value is comparatively hard to beat.

Carnival's Half Moon Cay itineraries are as affordable as $80 a day — not bad compared to neighboring private islands that could cost upward of $150,000 per night to rent.

But that doesn't mean every amenity at these cruise-owned destinations is cheap. If these savings give you the urge to splurge, a two-bedroom villa at Great Stirrup Cay's Silver Cove resort starts at about $1,430. Similarly, cabanas at CocoCay's luxurious beach club could cost upward of $1,600. (Don't worry — most of their beachside lounge chairs are free.)

The three cruise giants have hundreds of voyages planned to their islands in 2025. Carnival has about 200 scheduled — Royal Caribbean has more than double that.

These numbers are expected to grow in the coming years. Carnival and Norwegian plan to build multi-ship piers at their respective properties, with the latter opening in late 2025. This $150 million investment is expected to double Great Stirrup Cay's visitors the following year.