Cruises are a dreadful business during a global pandemic that stops mass gatherings from happening. A cruise -- even on a very large ship -- forces people into tight spaces where they are closely packed together. Rooms are small, ventilation is shared, and even outdoor spaces like pools tend to be packed with people.

That has put Royal Caribbean (RCL -1.45%), Carnival, and Norwegian Cruise Line (NCLH -0.42%) in a very tough spot. All three have had to take steps to raise money (at very steep prices), and none knows exactly when they will be able to begin operating (and what that will actually look like).

Uncertainty has beaten down the stock prices of all three major cruise lines. There are, however, some signs of hope -- bookings have been strong for next year, and tentative steps have been taken toward relaunching at least some ships when a CDC order banning cruises ends in late July.

Does that make any or all of the cruise lines a good investment? Maybe, but you have to know the risks.

A Royal Caribbean ship at sea.

Royal Caribbean has not cancelled its June and July sailings but they are no likely to happen. Image source: Royal Caribbean.

Where is the cruise industry now?

Royal Caribbean stock spiked by 16.69% on May 18 due to optimism over a potential COVID-19 vaccine. That still left its shares trading at $43.70, about a third off of their 52-week high. Carnival and Norwegian experienced similar share price jumps, but both also remain well below their 52-week highs.

The potential vaccine news was encouraging, but it's important to remember that it was a first-phase trial involving only 45 people. Good early results do not mean that the vaccine will work, and multiple steps remain to prove that it's both safe and effective.

Even if the vaccine works, it's not likely to be widely available until the end of the year. That means that the cruise industry faces significant operational hurdles if it hopes to begin even modified operations later this summer.

Carnival has been the most aggressive cruise line in pursuing a return. It has plans to sail 8 ships out of three ports beginning in August. That's a modest start, but even that limited offering comes with a lot of questions. It's easy enough to screen passengers heavily before they board. Additional cleaning can be added as well -- but social distancing on a cruise ship seems very difficult. It's somewhat possible to employ some distancing measures indoors on less-than-full ships. That's much harder on pool decks that are generally crowded even during slower times.

Will ships sail half full? That certainly won't be profitable, but it may help the cruise lines cut losses from being fully idle (or it may increase them).

Sunlight is generally believed to degrade COVID-19 quickly, but will that keep passengers safe outside? What happens if someone shows symptoms while onboard? What it someone is flagged as potentially being ill during the boarding process?

At the moment, not all of these questions can be answered. That leaves any potential restart of the industry uncertain at best.

Is now the time to buy cruise line stocks?

Buying shares of companies in an industry that has no clear path to opening is a huge risk. All three of the major cruise lines have raised billions of dollars in order to get through the year, but bankruptcies remain possible if cruising can't return.

If Carnival, Royal Caribbean, or Norwegian chooses to file Chapter 11 bankruptcy, they won't go away. They will, however, reorganize their finances in a way that wipes out shareholder equity.

Buying shares in any cruise line is a major risk. That risk comes with some major potential long-term upside, but if you buy you have to understand that it's going to take years of normal operations to dig out of debt. That could happen -- but so could bankruptcy.

All three cruise lines are smart buys as a tiny percentage of your portfolio, as long as you understand the time frame and the risks associated.