This seems to be Star Trek: Starfleet Corps of Engineers - The Belly of the Beast, published in 2000.
Here are some excerpts that fit the mentioned story points.
Regarding the ship's size and form:
It was a monster, more than fifty times bigger than the Enterprise...
It was round, like a small moon ... Two smooth rings circled the outer hull of the ship ... The rings were as thick as the Enterprise saucer section and twice as wide ... Picard had no idea what the rings were for.
Remote scaning and discovering that it's probably a cruise ship:
As they gathered details about the Beast, the computer would fill in
the three-dimensional map floating above the table surface ...
"We're getting clear images of the top deck area," Stevens said. "And
the rings. But the deeper we go into the center of the ship, the worse
Geordi could tell it was one amazing ship. There had to be far over a
hundred decks, with large open areas scattered throughout. And the
rings looked more like observation decks than anything else. There
were a number of very large, multiroom private quarters scattered in
the rings, and a lot of large gathering areas.
"Check out the material on the outer side of the rings," Geordi said,
pointing. "I'm betting you can see through it from the inside."
"What the hell was this ship?" Gomez asked.
"I'm putting my money on a cruise ship of some sort," Carol said.
Geordi knew instantly she was right ...
Less luxurious quarters on lower levels, and the strange readings of the core:
The room they had beamed into was large and well-lit, but not as
plushly furnished as the upper decks.
"Looks like this ship had some economy-class passenger fares as well,"
"You didn't expect all the rooms to look like those suites up in the
rings, did you?" Gomez asked.
Here there was no soft surface, no art, just door after door on both
sides of the halllway.
"Crew's quarters," Geordi said, studying his tricorder. "But there are
energy signatures coming from ten decks down. I think that might be a
warp core I'm reading." ...
"Getting weird readings," Geordi said frowning. "It seems to be
hollow, more than likely a null-gravity core of some sort. And it's
packed loosely with some sort of substance I'm not getting a fix on.
We need to get closer."
Once they reach the core, they find the dead crew and passengers:
The race that had inhabited this cruise ship was clearly humanoid in
almost every fashion ... They were packed into what had to be a core
area big enough to hold the Enterprise. They were all floating, limbs
tangled up in limbs, faces moving slowly past the viewport ... Faces frozen in terror and pain ... It was like a giant, slowly moving, zero-g dance of bodies.