(CNN) When you hear the phrase "Scientology cruise ship," it may conjure images of Tom Cruise playing shuffleboard on the deck while church members sunbathe and sip strawberry daiquiris.

That's not exactly what happens aboard the Freewinds.

The Freewinds, a cruise ship belonging to the Church of Scientology, was quarantined this week in the Caribbean with an apparent case of measles. It was allowed to return to Curacao harbor, its home port, on Saturday morning.

This has led to a number of questions, including: Wait, why does Scientology have a cruise ship?

According to the church, the Freewinds is less a vacation and more of a floating religious retreat center. Some former Scientologists have described it in harsher terms, but more on that later.

Once upon a time, Scientology had a fleet of ships, the church says, which were manned by its Sea Organization. The ships and crew helped Scientology founder L. Ron Hubbard with "research" and supervised church functions around the world, according to the church.

Scientologists compare the "Sea Org" to members of a religious order, like monks and nuns, who devote their lives to the faith, often working long hours for no pay and living communally. The Freewinds is entirely staffed by the Sea Org, the church says, who dress in spiffy naval uniforms.

But there was another reason for Scientologists' seafaring ways, say scholars who have studied the church. In the late 1960s several countries, including the United States, started scrutinizing Hubbard's new movement. He had been kicked out of England in 1968, according to Hugh Urban, author of " The Church of Scientology: A History of a New Religion.

"Thus Hubbard's shift to a sea-based organization during these years was clearly in part a response to his inability to operate freely in many nations," Urban writes.

CNN has reached out to the Church of Scientology but has not heard back from its communications department.

On the high seas, Scientologists were pretty much free do what they wanted, and the floating religious retreat centers eventually became an essential part of their practice.

In 1985, the church reportedly purchased the Finnish-built Freewinds, which once entertained tourists on the Commodore Cruise Line. The ship has since become its flagship, spiritually speaking, and it's mission is pretty ambitious: "The Freewinds is like no other place on Earth. It truly marks the beginning of a voyage to all eternity," the church says.

So what happens on the Freewinds?

Well, besides hosting a birthday party for Cruise, during which he grabbed the mic for an, um, energetic version of "Old Time Rock n Roll," what happens is a lot of intensive religious study. A church website describes the ship as a "safe, aesthetic, distraction-free environment appropriate for ministration of this profoundly spiritual level of auditing."

Auditing, for Scientologists, is a practice by which an individual can rid himself of " spiritual disabilities. " Holding a device called an e-meter, which functions as a kind of lie detector, the auditor interrogates the auditee, looking for areas of "spiritual distress."

The church describes the Freewinds as a kind of moving Mount of Olives, the site of Jesus' ascension into heaven. It's the place where Scientologists can reach the highest level of spiritual attainment, OT VIII, which stands for Operating Thetan Level 8. What, exactly, that entails is the subject of some mystery (and a little suspicion) outside of Scientology, but the church describes it as "the pinnacle of a deeply spiritual journey."

"Years of training and auditing have brought him to this ultimate point. It is the most significant spiritual accomplishment of his lifetime and brings with it the full realization of his immortality," the church says.

It's worth noting, though, that some former members have far less positive views of what goes on aboard the Freewinds.

In 2011, an Australian woman said she was taken aboard the ship for what she thought was a two-week vacation. But it turned into a 12-year-long form of indentured servitude, the woman alleged.

This week, actress Leah Remini, a former Scientologist who has become a fierce critic of the church, lodged similar charges about the Freewinds on Twitter. Responding to a Newsweek article about the ship, she said, "This is just the tip of the iceberg for what staff members of The Freewinds, Scientology's ship of horrors, have to endure while serving people like Tom Cruise & David Miscavige," the church's current leader.

Thank you @Newsweek @JenniFink_

for starting this conversation. This is just the tip of the iceberg for what staff members of The Freewinds, Scientology's ship of horrors, have to endure while serving people like Tom Cruise & David Miscavige https://t.co/q8CBm9rAvw — Leah Remini (@LeahRemini) May 2, 2019

Remini also said that the apparent measles outbreak this week aboard the Freewinds poses a challenge to Scientologists' beliefs about their supernatural powers.

The Scientology ship, The Freewinds, is where they reach one of the highest levels of Scientology & are supposed to be impervious to "Wog Illness". A Wog is a derogatory term used to describe all of you, who are all just average humans compared to the superior scientologist. — Leah Remini (@LeahRemini) May 2, 2019

"The Scientology ship, The Freewinds, is where they reach one of the highest levels of Scientology & are supposed to be impervious to "Wog Illness,' " Remini wrote. "A Wog is a derogatory term used to describe all of you, who are all just average humans compared to the superior Scientologist."