• Cruise lines advertise several promotions from January through March, known as wave season.
• Many premium cruise lines are promoting discounted fares and complimentary credits.
• This article is part of "Well Charted," a series for travelers planning cruise-bound adventures.

If you want affordable cruise fares and enough onboard credit to indulge in a spa day, consider wave season your vacation-at-sea Black Friday.

The cruise industry rolls out several promotions from January to March, known as wave season. Depending on the cruise line, these offers could involve discounts, free cabin upgrades, or hundreds of dollars' worth of credit.

The potential savings have become catnip for seafaring travelers, especially as strong demand raises cruise fares. Jason Liberty, the president and CEO of Royal Caribbean Group, said on Tuesday that the company already had its strongest-ever wave-season booking week.

Deals such as 50% off Norwegian voyages and 60% off a second Royal Caribbean guest have already ended (though a Royal Caribbean spokesperson told Business Insider it would have more wave-season promotions through March). But there are still plenty of discounts to choose from on higher-end cruise lines.

Which cruise lines are offering wave-season deals?

Holland America and Princess, Carnival's premium brands, are advertising discounts, complimentary cabin upgrades, and free fares for third or fourth guests in the same cabin (though Holland America requires they be under 18).

Royal Caribbean Group's Celebrity Cruises is promoting discounted itineraries and a discount of up to 75% on a second guest.

If you prefer higher-end cruise lines, Azamara, Cunard, and Explora are offering hundreds of dollars' worth of onboard credits.

Seabourn is promoting up to $1,000 worth of credit in conjunction with 15% off select sailings, while Regent Seven Seas is advertising a 20% discount on certain itineraries, complimentary suite upgrades, and discounted deposits.

Other ways to save on your next cruise vacation

There are other ways to find a bargain if you couldn't surf this year's early wave-season deals (or are too impatient to wait for more).

Holland America's standby cruise deal starts at $100 per person per day — though your sailing won't be confirmed until two to seven days before departure.

Companies such as Costco Travel, Holland America, and Royal Caribbean also offer discounted last-minute voyages year-round.

Finally, you could use a travel agent who might be privy to promotions you don't know about.